[PATCHSET v9 sched_ext/for-7.3] sched: Make proxy execution compatible with sched_ext

[PATCHSET v9 sched_ext/for-7.3] sched: Make proxy execution compatible with sched_ext

[Andrea Righi]

This series enables using proxy execution with sched_ext and is based on early
work by John Stultz [1].

Background
==========

Proxy execution (proxy-exec) lets a waiting task ("donor") donate its scheduling
context to a mutex owner, so the owner can run while the donor stays eligible on
the runqueue.

Currently, proxy-exec and sched_ext are mutually exclusive at build time: we
can't enable CONFIG_SCHED_PROXY_EXEC=y and CONFIG_SCHED_CLASS_EXT=y in the same
kernel.

This restriction can be problematic for Linux distributions and for anyone who
wants to ship one kernel and choose features at runtime.

Why are they mutually exclusive?
================================

sched_ext schedulers drive dispatch through their own interfaces. A proxy-exec
handoff can run a task that the BPF scheduler never dispatched through that
path. sched_ext callbacks then observe a "current" task that does not match what
the BPF side considers running, so kfuncs and helper state can see an
inconsistent view of the executing task.

sched_ext also tracks runnable work through Dispatch Queues (DSQs) and BPF
chosen dispatch rules, while the core scheduler still maintains classic per-CPU
runqueues and pick paths. A proxy handoff can therefore switch the CPU to a task
that the BPF scheduler never inserted or ordered through its DSQ interface.

DSQ state, vtime, and "who is running" bookkeeping inside the BPF program can
then disagree with what the core actually executes, so helpers and kfuncs that
assume their dispatched task is current may observe stale or inconsistent state.

Design: supporting proxy-exec with sched_ext
============================================

Provide proxy-exec in sched_ext as optional per-scheduler capability: a BPF
scheduler can set the ops flag SCX_OPS_ENQ_BLOCKED to keep mutex-blocked donors
runnable and receive them through ops.enqueue() with SCX_ENQ_BLOCKED set.

This flag requires an ops.enqueue() callback and gives BPF control over whether,
where, and in what order to dispatch each donor. The core then walks the
mutex-owner chain and, when needed, migrates the donor's scheduling context to
the owner's rq before executing the owner.

Without the SCX_OPS_ENQ_BLOCKED flag, mutex waiters block normally and do not
participate in proxy-exec while owned by that scheduler.

Once BPF dispatches a donor, proxy execution may move its scheduling context
to the mutex owner's rq. This move is distinct from BPF placement: wake_cpu
continues to identify the donor's callback CPU, while task_cpu() identifies the
proxy CPU. When the donor wakes, the normal wakeup path uses wake_cpu as
prev_cpu when returning the task to the BPF scheduler, which can then choose
its next placement normally.

The donor-to-owner handoff is modeled like a "function call" from the
scheduler's perspective. The donor remains the running scheduling entity
selected by BPF: its scheduling context, runtime and slice are consumed while
the core temporarily invokes the mutex owner's code to make the critical section
progress. It is not a scheduler-visible switch to the owner.

Accordingly, the donor remains the scheduling context presented to sched_ext,
while the mutex owner is treated solely as the execution context selected
internally by the core scheduler. Scheduling state is accounted against
rq->donor where appropriate, while rq->curr identifies the execution context.
The internal owner substitution does not generate synthetic sched_ext callbacks
for a task that BPF did not dispatch.

Scheduler-facing current-task kfuncs follow the same rule:
scx_bpf_task_running(), scx_bpf_cpu_curr() and scx_bpf_cid_curr() report the
donor selected by the scheduler rather than the mutex owner whose code happens
to execute on its behalf.

The sched_ext callback bookkeeping is adjusted accordingly. A blocked donor
enters a tracked ops.running()/ops.stopping() session only after proxy
resolution succeeds, and ops.stopping() is only called for tasks that entered
such a session. This state is independent of physical rq->curr execution,
which may refer to the mutex owner instead of the donor.

NOHZ CFS bandwidth checks also follow rq->donor rather than rq->curr and use
the number of queued FAIR tasks when deciding whether the tick can stop. This
keeps bandwidth enforcement active for a constrained FAIR donor even when its
proxy owner remains runnable in another scheduling class.

Scheduler ownership changes need special handling because a donor may already be
blocked when a root scheduler is enabled, when a task moves between root and
sub-schedulers, or when PI de-boosting moves it into EXT. If the incoming
scheduler does not set SCX_OPS_ENQ_BLOCKED, the retained donor is removed from
the runqueue and stays on the normal mutex wait path. Otherwise it remains
eligible for BPF admission.

Remote DSQ transfers recheck proxy-sensitive state after locking the source
runqueue. If a transfer races with proxy execution, the task is parked on the
source runqueue's reject DSQ and returned to its owning BPF scheduler after
proxy resolution settles. This preserves scheduler placement and sub-scheduler
capability rules without migrating an active execution context.

scx_qmap has been modified with a -B option to enable queueing mutex-blocked
tasks for proxy-exec. Blocked donors receive a fresh slice, are inserted at the
head of their current cid's local DSQ and request immediate preemption. This is
an intentionally aggressive policy for making proxy-exec easy to observe.
Capability-rejection re-enqueues bypass this blocked-donor fast path and fall
back to the shared DSQ, avoiding repeated rejection on a cid that no longer has
the required capability. A new statistic reports blocked-donor dispatches.

A new kselftest (enq_blocked) is also introduced to validate proxy-exec with
sched_ext. The test creates a priority inversion with a low-priority owner
(nice +19), a high-priority donor (nice -20) and one nice 0 contender per
available CPU. It exercises a same-CPU topology and a cross-CPU topology, where
the donor and owner run on different CPUs. Each configuration runs with
SCX_OPS_ENQ_BLOCKED first disabled and then enabled, counts blocked-donor
enqueues by CPU and reports mutex hold/wait-time deltas. Access to the mutex is
provided by a loadable kernel module built via TEST_GEN_MODS_DIR, with the test
responsible for its lifecycle.

Example kselftest run:

===== START =====
TEST: enq_blocked
DESCRIPTION: Verify proxy donor admission under CPU-wide contention
OUTPUT:

[topology=same-cpu SCX_OPS_ENQ_BLOCKED=disabled]
  proxy_exec=enabled
  donor_cpu=0
  owner_cpu=0
  nr_contenders=16
  measured_trials=10
  owner_nice=19
  donor_nice=-20
  contender_nice=0
  mutex_hold_avg_ns=263587544 (263.587 ms, samples=10)
  mutex_wait_avg_ns=263597934 (263.597 ms, samples=10)
  nr_blocked_enqueues=0
  nr_blocked_enqueues_donor_cpu=0
  nr_blocked_enqueues_owner_cpu=0
  nr_blocked_enqueues_other_cpu=0
  nr_blocked_wakeups=0

[topology=same-cpu SCX_OPS_ENQ_BLOCKED=enabled]
  proxy_exec=enabled
  donor_cpu=0
  owner_cpu=0
  nr_contenders=16
  measured_trials=10
  owner_nice=19
  donor_nice=-20
  contender_nice=0
  mutex_hold_avg_ns=251789178 (251.789 ms, samples=10)
  mutex_wait_avg_ns=209808599 (209.808 ms, samples=10)
  nr_blocked_enqueues=60
  nr_blocked_enqueues_donor_cpu=60
  nr_blocked_enqueues_owner_cpu=0
  nr_blocked_enqueues_other_cpu=0
  nr_blocked_wakeups=0

[topology=same-cpu delta: enabled - disabled]
  mutex_hold_delta_ns=-11798366 (-4.48%)
  mutex_wait_delta_ns=-53789335 (-20.41%)

[topology=cross-cpu SCX_OPS_ENQ_BLOCKED=disabled]
  proxy_exec=enabled
  donor_cpu=0
  owner_cpu=1
  nr_contenders=16
  measured_trials=10
  owner_nice=19
  donor_nice=-20
  contender_nice=0
  mutex_hold_avg_ns=246794454 (246.794 ms, samples=10)
  mutex_wait_avg_ns=247298497 (247.298 ms, samples=10)
  nr_blocked_enqueues=0
  nr_blocked_enqueues_donor_cpu=0
  nr_blocked_enqueues_owner_cpu=0
  nr_blocked_enqueues_other_cpu=0
  nr_blocked_wakeups=0

[topology=cross-cpu SCX_OPS_ENQ_BLOCKED=enabled]
  proxy_exec=enabled
  donor_cpu=0
  owner_cpu=1
  nr_contenders=16
  measured_trials=10
  owner_nice=19
  donor_nice=-20
  contender_nice=0
  mutex_hold_avg_ns=215486809 (215.486 ms, samples=10)
  mutex_wait_avg_ns=215498090 (215.498 ms, samples=10)
  nr_blocked_enqueues=80
  nr_blocked_enqueues_donor_cpu=20
  nr_blocked_enqueues_owner_cpu=60
  nr_blocked_enqueues_other_cpu=0
  nr_blocked_wakeups=0

[topology=cross-cpu delta: enabled - disabled]
  mutex_hold_delta_ns=-31307645 (-12.69%)
  mutex_wait_delta_ns=-31800407 (-12.86%)
ok 1 enq_blocked #
=====  END  =====


=============================

RESULTS:

PASSED:  1
SKIPPED: 0
FAILED:  0

References
==========

[1] https://lore.kernel.org/all/20251206001451.1418225-1-jstultz@google.com

Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arighi/linux.git scx-proxy-exec

Changes in v9:
 - Add an incoming-class prepare_switch() callback and use it to block retained
   donors before transitions into sched_ext (Tejun Heo)
 - Generalize the per-rq reject DSQ and use it for proxy-raced remote transfers
   instead of bouncing tasks to a global DSQ (Tejun Heo)
 - Restrict migration-disabled warning exception to blocked donors whose
   scheduling context was actually proxy-migrated (sashiko)
 - Keep ops.tick() and other donor accounting inside a tracked
   ops.running()/ops.stopping() session (sashiko)
 - Prevent BPF-directed migration of an active proxy donor and use the donor as
   the current scheduling context during restore (sashiko)
 - Update sched_fair_update_stop_tick() to use the number of queued FAIR tasks
   (sashiko)
 - Fold the temporary local-DSQ donor placement into the final BPF-controlled
   admission change
 - Link to v8: https://lore.kernel.org/all/20260721063242.552774-1-arighi@nvidia.com/

Changes in v8:
 - Rework remote-DSQ migration check so the locked re-check handles only
   proxy-exec state changes (Tejun Heo)
 - Use the current-donor helper when preventing migration of an active donor
   (sashiko)
 - Handle blocked-donor reactivation into scx schedulers without
   SCX_OPS_ENQ_BLOCKED (John Stultz)
 - Document sched_ext callback behavior under proxy execution (sashiko)
 - Avoid redispatching a rejected blocked donor to the same local DSQ in
   scx_qmap (sashiko)
 - Add a preparatory fix to avoid false migration warnings when proxy-exec moves
   a migration-disabled donor
 - Skip the enq_blocked kselftest when proxy execution is unavailable
 - Link to v7: https://lore.kernel.org/all/20260716132229.61603-1-arighi@nvidia.com/

Changes in v7:
 - Move the remote CPU check out of the lockless remote-DSQ scan and perform it
   only after locking the source rq, retain the locked migration recheck
   immediately before moving the task (sashiko)
 - Complete the scheduler-context conversion from rq->curr to rq->donor for
   capability revocation and preemption checks, make scx_bpf_task_running(),
   scx_bpf_cpu_curr() and scx_bpf_cid_curr() expose the donor selected by the
   scheduler (sashiko)
 - Handle SCX_ENQ_REENQ before scx_qmap's blocked-donor fast path to avoid an
   infinite reject/re-enqueue loop (sashiko)
 - Add scx_qmap stat for SCX_ENQ_BLOCKED dispatches
 - Link to v6: https://lore.kernel.org/all/20260715205622.276220-1-arighi@nvidia.com/

Changes in v6:
 - Add a scheduler-core fix to make NOHZ CFS bandwidth checks follow the proxy
   donor instead of the physical execution context (sashiko)
 - Block retained donors when entering sched_ext through PI de-boosting or
   global scheduler activation (sashiko)
 - Distinguish ordinary wakeup activations from retained-donor admissions so
   SCX_ENQ_BLOCKED is not reported for normal wakeups; extend the selftest to
   detect this regression (sashiko)
 - Validate unexpected-CPU blocked-donor enqueues in both same-CPU and
   cross-CPU test topologies (sashiko)
 - Link to v5: https://lore.kernel.org/all/20260713162112.26785-1-arighi@nvidia.com/

Changes in v5:
 - Split retained-donor deactivation and sched_ext's default rejection into
   preparatory patches so the scheduler-core changes can be routed separately
 - Drop the proxy destination query kfuncs and the preparatory mutex lock-scope
   change (John Stultz)
 - Use p->is_blocked instead of task_is_blocked() to fix a WARN triggered during
   scx_pair testing (John Stultz)
 - Rename SCX_TASK_IS_RUNNING to SCX_TASK_RUN_TRACKED and document that it
   tracks an ops.running()/stopping() session rather than physical rq->curr
   execution
 - Keep a proxy-migrated blocked donor on the owner's rq until wakeup instead of
   allowing BPF-directed migration to pull it back to wake_cpu and cause
   repeated donor migration
 - Extend kselftest to cover same-CPU and cross-CPU proxy-exec switches
 - Make scx_qmap insert blocked donors at the head of their current cid's local
   DSQ and request immediate preemption
 - Link to v4: https://lore.kernel.org/all/20260710083913.30573-1-arighi@nvidia.com/

Changes in v4:
 - Harden scx_bpf_task_proxy_cpu() locking and blocked-state validation, return
   -ENOENT for unrunnable owners and drop unnecessary donor-affinity checks
   (K Prateek Nayak, sashiko)
 - Reschedule remote CPUs after donor deactivation, handle sched_setscheduler()
   admission and assert scheduler-change locking (sashiko)
 - Avoid a potential KCSAN data-race report in the lockless blocked-donor
   migration check by using rcu_access_pointer() for rq->donor (sashiko)
 - Fix tick dependency updates for incoming EXT contexts and keep the tick
   enabled for blocked donors (sashiko)
 - Dump EXT donors in scx_dump_state() (sashiko)
 - Reordered the preparatory sched_ext changes so real running-state tracking
   is established before donor-based accounting, and split the proxy
   destination query kfuncs into a separate patch
 - Add compatibility wrappers for the proxy CPU/cid kfuncs and document their
   results as scheduling hints
 - Link to v3: https://lore.kernel.org/all/20260706070410.282826-1-arighi@nvidia.com/

Changes in v3:
 - Dropped the core restrictions on proxy-migrating migration-disabled and
   single-CPU donors: proxy execution moves the scheduling context, not the
   task's execution context. (Peter Zijlstra, K Prateek Nayak)
 - Dropped the sched_ext-specific put_prev_task()/set_next_task() exception and
   fixed ops.running()/ops.stopping() pairing inside sched_ext instead; track a
   real running transition even when either callback is absent. (Peter Zijlstra)
 - Dropped the kf_tasks[] nesting and nested ops.runnable() patches from v2;
   extensive proxy-exec testing did not reproduce task-op re-entry, so retain
   the existing non-nesting invariant.
 - Replaced scx_bpf_task_is_blocked() with SCX_ENQ_BLOCKED in ops.enqueue()
   flags, identifying blocked-donor admission directly at enqueue time.
 - Expanded the curr/donor description to cover mixed scheduling classes and
   fixed local preemption to expire rq->donor's slice. (Aiqun Maria Yu)
 - Allow inactive blocked donors to be placed on a remote local DSQ while
   preventing normal migration of an active rq donor. (Aiqun Maria Yu)
 - Deactivate retained donors when ownership changes to a root or sub-scheduler
   without SCX_OPS_ENQ_BLOCKED, and extend the selftest to cover attaching a
   scheduler after the donor blocks. (K Prateek Nayak)
 - Harden remote DSQ consumption by rejecting active tasks and rechecking
   migration eligibility after locking the source rq. Fall back to the global
   DSQ without treating an eligibility change during the lock handoff as a BPF
   scheduler error.
 - scx_qmap has a command line option (-B) to enable blocked-donor queueing
 - Link to v2: https://lore.kernel.org/all/20260702171909.1994478-1-arighi@nvidia.com/

Changes in v2:
 - Rebased onto sched_ext/for-7.3 and adapted the series to the split
   sched_ext implementation and cid-form scheduler interfaces.
 - Replaced the global sched_proxy_exec_scx boot-time opt-in with the
   per-scheduler SCX_OPS_ENQ_BLOCKED capability, allowing BPF to control donor
   admission and ordering through ops.enqueue().
 - Added scx_bpf_task_is_blocked(), scx_bpf_task_proxy_cpu(), and
   scx_bpf_task_proxy_cid(); enforce CPU/cid API separation for cid-form
   schedulers.
 - Added proxy exec support to scx_qmap, including optional owner-cid steering,
   affinity validation, and fallback to the donor's current cid.
 - Added a kselftest with a kernel mutex test and a three-task priority
   inversion workload, test is executed with blocked task admission disabled and
   enabled, validates the behavior, and reports hold/wait-time deltas.
 - Link to v1: https://lore.kernel.org/all/20260506174639.535232-1-arighi@nvidia.com/

Andrea Righi (14):
      sched/core: Avoid false migration warning for proxy donors
      sched: Make NOHZ CFS bandwidth checks follow proxy donor
      sched: Add helper to block retained proxy donors
      sched: Add prepare_switch() class callback
      sched: Add sched_ext hooks for proxy execution
      sched_ext: Block proxy donors across scheduler transitions
      sched_ext: Fix ops.running/stopping() pairing for proxy-exec donors
      sched_ext: Generalize the reject DSQ reenqueue path
      sched_ext: Handle proxy-exec races in remote DSQ transfers
      sched_ext: Split curr|donor references properly
      sched_ext: Delegate proxy donor admission to BPF schedulers
      sched_ext: Add selftest for blocked donor admission
      sched_ext: scx_qmap: Add proxy execution support
      sched: Allow enabling proxy exec with sched_ext

 Documentation/scheduler/sched-ext.rst              |   6 +
 include/linux/sched/ext.h                          |   8 +
 init/Kconfig                                       |   2 -
 kernel/sched/core.c                                |  76 +-
 kernel/sched/ext/ext.c                             | 550 ++++++++++--
 kernel/sched/ext/ext.h                             |   6 +
 kernel/sched/ext/internal.h                        |  29 +-
 kernel/sched/ext/sub.c                             |  58 +-
 kernel/sched/ext/sub.h                             |  10 +-
 kernel/sched/fair.c                                |  12 +-
 kernel/sched/sched.h                               |  21 +-
 kernel/sched/syscalls.c                            |   4 +-
 tools/sched_ext/include/scx/compat.h               |   1 +
 tools/sched_ext/include/scx/enum_defs.autogen.h    |   1 +
 tools/sched_ext/include/scx/enums.autogen.bpf.h    |   3 +
 tools/sched_ext/include/scx/enums.autogen.h        |   1 +
 tools/sched_ext/scx_qmap.bpf.c                     |  34 +
 tools/sched_ext/scx_qmap.c                         |  13 +-
 tools/sched_ext/scx_qmap.h                         |   1 +
 tools/testing/selftests/sched_ext/.gitignore       |   4 +
 tools/testing/selftests/sched_ext/Makefile         |   2 +
 tools/testing/selftests/sched_ext/config           |   2 +
 .../testing/selftests/sched_ext/enq_blocked.bpf.c  | 116 +++
 tools/testing/selftests/sched_ext/enq_blocked.c    | 918 +++++++++++++++++++++
 tools/testing/selftests/sched_ext/enq_blocked.h    |  28 +
 .../selftests/sched_ext/test_modules/Makefile      |  13 +
 .../sched_ext/test_modules/scx_enq_blocked_test.c  | 195 +++++
 27 files changed, 1939 insertions(+), 175 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.c
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.h
 create mode 100644 tools/testing/selftests/sched_ext/test_modules/Makefile
 create mode 100644 tools/testing/selftests/sched_ext/test_modules/scx_enq_blocked_test.c

[PATCH 01/14] sched/core: Avoid false migration warning for proxy donors

[Andrea Righi]

Proxy execution can move a blocked donor's scheduling context to the
lock owner's CPU even when the donor is migration-disabled. The donor
does not execute there, and its original execution CPU remains recorded
in wake_cpu.

set_task_cpu() warns unconditionally for migration-disabled tasks, so a
subsequent proxy migration or the wakeup path returning the donor home
triggers a false positive. Moving a blocked scheduling context does not
violate the migration-disabled execution context.

Exclude blocked proxy donors from the warning. The proxy wakeup path
restores an executable placement before clearing the blocked state.

This is a preparatory fix for enabling proxy execution together with
sched_ext.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3cc6fb1d20547..e11fac7f805e7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3341,6 +3341,8 @@ void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 {
 	unsigned int state = READ_ONCE(p->__state);
+	bool proxy_migrated = sched_proxy_exec() && p->is_blocked &&
+			      task_cpu(p) != p->wake_cpu;
 
 	/*
 	 * We should never call set_task_cpu() on a blocked task,
@@ -3376,7 +3378,12 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 	 */
 	WARN_ON_ONCE(!cpu_online(new_cpu));
 
-	WARN_ON_ONCE(is_migration_disabled(p));
+	/*
+	 * Proxy execution can move a blocked task's scheduling context to any
+	 * CPU without moving its migration-disabled execution context. The
+	 * wakeup path will return the task to a CPU where it can execute.
+	 */
+	WARN_ON_ONCE(is_migration_disabled(p) && !proxy_migrated);
 
 	trace_sched_migrate_task(p, new_cpu);
 
-- 
2.55.0

[PATCH 02/14] sched: Make NOHZ CFS bandwidth checks follow proxy donor

[Andrea Righi]

Proxy execution separates the scheduling context in rq->donor from the
physical execution context in rq->curr. sched_can_stop_tick() checks the
latter for CFS bandwidth constraints and only does so when nr_running is
one.

A retained proxy donor keeps both the donor and mutex owner queued. The
check therefore misses a constrained FAIR donor and may stop the tick
while its runtime still needs to be enforced.

Check the selected donor instead and remove the nr_running restriction.
The donor being a queued FAIR task is sufficient to require bandwidth
accounting regardless of other runnable tasks.

Fixes: af0c8b2bf67b ("sched: Split scheduler and execution contexts")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/r/20260713164807.E5ED21F00A3A@smtp.kernel.org
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c | 18 +++++++++---------
 kernel/sched/fair.c | 12 +++++++-----
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e11fac7f805e7..e9829a289da45 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1409,11 +1409,8 @@ static void nohz_csd_func(void *info)
 #endif /* CONFIG_NO_HZ_COMMON */
 
 #ifdef CONFIG_NO_HZ_FULL
-static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
+static inline bool __need_bw_check(struct task_struct *p)
 {
-	if (rq->nr_running != 1)
-		return false;
-
 	if (p->sched_class != &fair_sched_class)
 		return false;
 
@@ -1462,14 +1459,17 @@ bool sched_can_stop_tick(struct rq *rq)
 		return false;
 
 	/*
-	 * If there is one task and it has CFS runtime bandwidth constraints
-	 * and it's on the cpu now we don't want to stop the tick.
+	 * If the selected scheduling context has CFS runtime bandwidth
+	 * constraints, don't stop the tick. With proxy execution, rq->donor is
+	 * the selected scheduling context while rq->curr is the task physically
+	 * executing on its behalf.
+	 *
 	 * This check prevents clearing the bit if a newly enqueued task here is
-	 * dequeued by migrating while the constrained task continues to run.
+	 * dequeued by migrating while the constrained donor continues to run.
 	 * E.g. going from 2->1 without going through pick_next_task().
 	 */
-	if (__need_bw_check(rq, rq->curr)) {
-		if (cfs_task_bw_constrained(rq->curr))
+	if (__need_bw_check(rq->donor)) {
+		if (cfs_task_bw_constrained(rq->donor))
 			return false;
 	}
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee13..14c5b2aa01777 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7570,14 +7570,16 @@ static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
 
-	if (rq->nr_running != 1)
+	if (rq->cfs.h_nr_queued != 1)
 		return;
 
 	/*
-	 *  We know there is only one task runnable and we've just picked it. The
-	 *  normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
-	 *  be otherwise able to stop the tick. Just need to check if we are using
-	 *  bandwidth control.
+	 * We know there is only one FAIR task queued and we've just picked it.
+	 * The normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we
+	 * will be otherwise able to stop the tick. With proxy execution, its
+	 * lock owner may remain runnable in another scheduling class, so
+	 * rq->nr_running can be greater than one. Just need to check if we are
+	 * using bandwidth control.
 	 */
 	if (cfs_task_bw_constrained(p))
 		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
-- 
2.55.0

[PATCH 03/14] sched: Add helper to block retained proxy donors

[Andrea Righi]

Scheduler ownership changes may need to turn a mutex-blocked task
retained on the runqueue for proxy execution back into a normally
blocked task.

Add sched_proxy_block_task() to perform that transition while holding
p->pi_lock and the task's runqueue lock. Reset an active donor to the
physical current task before blocking it. If DELAY_DEQUEUE keeps a FAIR
donor queued, the immediately following sched_change_begin() completes
the dequeue through switching_from_fair() before recording its queued
state.

This is a preparatory change to support proxy execution with sched_ext.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c  | 28 ++++++++++++++++++++++++++++
 kernel/sched/sched.h |  6 ++++++
 2 files changed, 34 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e9829a289da45..9a9d4b5040089 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6771,6 +6771,34 @@ static void proxy_deactivate(struct rq *rq, struct task_struct *donor)
 	block_task(rq, donor, state);
 }
 
+/*
+ * Remove a retained proxy donor before changing its scheduler ownership.
+ * The caller holds p->pi_lock, so p cannot wake and migrate if block_task()
+ * drops it from the runqueue. If DELAY_DEQUEUE keeps a blocked fair task
+ * queued, switching_from_fair() completes the dequeue in the immediately
+ * following sched_change_begin().
+ */
+void sched_proxy_block_task(struct rq *rq, struct task_struct *p)
+{
+	unsigned long state = READ_ONCE(p->__state);
+
+	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_rq_held(rq);
+
+	if (!p->is_blocked || !task_on_rq_queued(p))
+		return;
+	if (WARN_ON_ONCE(state == TASK_RUNNING))
+		return;
+
+	if (task_current_donor(rq, p))
+		proxy_reset_donor(rq);
+
+	if (!p->se.sched_delayed)
+		block_task(rq, p, state);
+
+	WARN_ON_ONCE(task_on_rq_queued(p) && !p->se.sched_delayed);
+}
+
 static inline void proxy_release_rq_lock(struct rq *rq, struct rq_flags *rf)
 	__releases(__rq_lockp(rq))
 {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 64d79e9efc3d1..437c4f68c9e53 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2477,6 +2477,12 @@ static inline bool task_is_blocked(struct task_struct *p)
 	return !!p->blocked_on;
 }
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+void sched_proxy_block_task(struct rq *rq, struct task_struct *p);
+#else
+static inline void sched_proxy_block_task(struct rq *rq, struct task_struct *p) {}
+#endif
+
 static inline int task_on_cpu(struct rq *rq, struct task_struct *p)
 {
 	return p->on_cpu;
-- 
2.55.0

[PATCH 04/14] sched: Add prepare_switch() class callback

[Andrea Righi]

sched_change_begin() only invokes callbacks on the outgoing class before
recording the task queueing state. An incoming class may need to prepare
a task while that state is still live, before the normal dequeue and
class transition take place.

Pass the incoming class to sched_change_begin() and invoke its optional
prepare_switch() callback before recording the queued and running state.

This is a preparatory change to support proxy execution with sched_ext.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c     | 16 +++++++++++-----
 kernel/sched/ext/ext.c  |  7 ++++---
 kernel/sched/ext/sub.c  |  9 ++++++---
 kernel/sched/sched.h    | 10 +++++++---
 kernel/sched/syscalls.c |  4 ++--
 5 files changed, 30 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a9d4b5040089..474628f520a29 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2792,7 +2792,7 @@ void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx
 static void
 do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
 {
-	scoped_guard (sched_change, p, DEQUEUE_SAVE)
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE)
 		p->sched_class->set_cpus_allowed(p, ctx);
 }
 
@@ -7722,7 +7722,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 	if (prev_class != next_class)
 		queue_flag |= DEQUEUE_CLASS;
 
-	scoped_guard (sched_change, p, queue_flag) {
+	scoped_guard (sched_change, p, next_class, queue_flag) {
 		/*
 		 * Boosting condition are:
 		 * 1. -rt task is running and holds mutex A
@@ -8399,7 +8399,7 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
 void sched_setnuma(struct task_struct *p, int nid)
 {
 	guard(task_rq_lock)(p);
-	scoped_guard (sched_change, p, DEQUEUE_SAVE)
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE)
 		p->numa_preferred_nid = nid;
 }
 #endif /* CONFIG_NUMA_BALANCING */
@@ -9516,7 +9516,7 @@ void sched_move_task(struct task_struct *tsk, bool for_autogroup)
 	CLASS(task_rq_lock, rq_guard)(tsk);
 	rq = rq_guard.rq;
 
-	scoped_guard (sched_change, tsk, queue_flags) {
+	scoped_guard (sched_change, tsk, tsk->sched_class, queue_flags) {
 		sched_change_group(tsk);
 		if (!for_autogroup)
 			scx_cgroup_move_task(tsk);
@@ -11220,7 +11220,9 @@ static inline void sched_mm_cid_fork(struct task_struct *t) { }
 
 static DEFINE_PER_CPU(struct sched_change_ctx, sched_change_ctx);
 
-struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags)
+struct sched_change_ctx *
+sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
+		   unsigned int flags)
 {
 	struct sched_change_ctx *ctx = this_cpu_ptr(&sched_change_ctx);
 	struct rq *rq = task_rq(p);
@@ -11238,6 +11240,10 @@ struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int
 		flags |= DEQUEUE_NOCLOCK;
 	}
 
+	if ((flags & DEQUEUE_CLASS) && next_class != p->sched_class &&
+	    next_class->prepare_switch)
+		next_class->prepare_switch(rq, p);
+
 	if ((flags & DEQUEUE_CLASS) && p->sched_class->switching_from)
 		p->sched_class->switching_from(rq, p);
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 81c2d8eeae410..285bd8422193d 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -5859,7 +5859,8 @@ void scx_bypass(struct scx_sched *sch, bool bypass)
 				continue;
 
 			/* cycling deq/enq is enough, see the function comment */
-			scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+			scoped_guard (sched_change, p, p->sched_class,
+				      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 				/* nothing */ ;
 			}
 		}
@@ -6141,7 +6142,7 @@ static void scx_root_disable(struct scx_sched *sch)
 		if (old_class != new_class)
 			queue_flags |= DEQUEUE_CLASS;
 
-		scoped_guard (sched_change, p, queue_flags) {
+		scoped_guard (sched_change, p, new_class, queue_flags) {
 			p->sched_class = new_class;
 		}
 
@@ -7477,7 +7478,7 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 		if (old_class != new_class)
 			queue_flags |= DEQUEUE_CLASS;
 
-		scoped_guard (sched_change, p, queue_flags) {
+		scoped_guard (sched_change, p, new_class, queue_flags) {
 			set_task_slice(p, READ_ONCE(sch->slice_dfl));
 			p->sched_class = new_class;
 		}
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 6da6c91e42870..765b516e06c95 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -791,7 +791,8 @@ static void scx_rehome_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_held(&p->pi_lock);
 	lockdep_assert_rq_held(task_rq(p));
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+	scoped_guard (sched_change, p, p->sched_class,
+		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_state(p, SCX_TASK_INIT_BEGIN);
 		scx_set_task_state(p, SCX_TASK_INIT);
@@ -821,7 +822,8 @@ static void scx_punt_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_rq_held(task_rq(p));
 	WARN_ON_ONCE(!READ_ONCE(to->bypass_depth));
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+	scoped_guard (sched_change, p, p->sched_class,
+		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_sched(p, to);
 	}
@@ -1471,7 +1473,8 @@ void scx_sub_enable_workfn(struct kthread_work *work)
 		if (!(p->scx.flags & SCX_TASK_SUB_INIT))
 			continue;
 
-		scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+		scoped_guard (sched_change, p, p->sched_class,
+			      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 			/*
 			 * $p must be either READY or ENABLED. If ENABLED,
 			 * __scx_disabled_and_exit_task() first disables and
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 437c4f68c9e53..ca436a8174d30 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2705,6 +2705,7 @@ struct sched_class {
 	/*
 	 * sched_change
 	 */
+	void (*prepare_switch)(struct rq *this_rq, struct task_struct *task);
 	void (*switching_from)(struct rq *this_rq, struct task_struct *task);
 	void (*switched_from) (struct rq *this_rq, struct task_struct *task);
 	void (*switching_to)  (struct rq *this_rq, struct task_struct *task);
@@ -4214,13 +4215,16 @@ struct sched_change_ctx {
 	bool			running;
 };
 
-struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags);
+struct sched_change_ctx *
+sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
+		   unsigned int flags);
 void sched_change_end(struct sched_change_ctx *ctx);
 
 DEFINE_CLASS(sched_change, struct sched_change_ctx *,
 	     sched_change_end(_T),
-	     sched_change_begin(p, flags),
-	     struct task_struct *p, unsigned int flags)
+	     sched_change_begin(p, next_class, flags),
+	     struct task_struct *p, const struct sched_class *next_class,
+	     unsigned int flags)
 
 DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
 
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index b215b0ead9a60..bc32ce76ff4fe 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -85,7 +85,7 @@ void set_user_nice(struct task_struct *p, long nice)
 		return;
 	}
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE) {
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE) {
 		p->static_prio = NICE_TO_PRIO(nice);
 		set_load_weight(p, true);
 		old_prio = p->prio;
@@ -678,7 +678,7 @@ int __sched_setscheduler(struct task_struct *p,
 	if (prev_class != next_class)
 		queue_flags |= DEQUEUE_CLASS;
 
-	scoped_guard (sched_change, p, queue_flags) {
+	scoped_guard (sched_change, p, next_class, queue_flags) {
 
 		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
 			__setscheduler_params(p, attr);
-- 
2.55.0

[PATCH 05/14] sched: Add sched_ext hooks for proxy execution

[Andrea Righi]

Proxy execution splits the scheduling context (the donor) from the
execution context (the lock owner). sched_ext needs to observe that
split at three points in __schedule():

 - whether a blocked EXT task can be retained on the runqueue as a
   donor,
 - when a donor's scheduling context starts driving a lock owner,
 - once proxy resolution has settled for the current pick.

Introduce scx_allow_proxy_exec(), scx_proxy_donor_start() and
scx_proxy_resolved(), and add their call sites in __schedule(). The
implementations are empty here and are filled in by the sched_ext
changes that follow, so that all the sched core changes needed by proxy
execution stay together in the preparatory patches.

SCHED_PROXY_EXEC still depends on !SCHED_CLASS_EXT, so the new hooks are
inert: they are compiled out with CONFIG_SCHED_CLASS_EXT=n and
unreachable otherwise.

This is a preparatory change to support proxy execution with sched_ext.
No functional change.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c    |  5 ++++-
 kernel/sched/ext/ext.c | 13 +++++++++++++
 kernel/sched/ext/ext.h |  6 ++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 474628f520a29..2018c4eb680a1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7169,7 +7169,7 @@ static void __sched notrace __schedule(int sched_mode)
 		 * task_is_blocked() will always be false).
 		 */
 		try_to_block_task(rq, prev, &prev_state,
-				  !task_is_blocked(prev));
+				  !task_is_blocked(prev) || !scx_allow_proxy_exec(prev));
 		switch_count = &prev->nvcsw;
 	}
 
@@ -7190,6 +7190,7 @@ static void __sched notrace __schedule(int sched_mode)
 			}
 			if (next == rq->idle) {
 				zap_balance_callbacks(rq);
+				scx_proxy_resolved(rq);
 				goto keep_resched;
 			}
 		}
@@ -7210,6 +7211,8 @@ static void __sched notrace __schedule(int sched_mode)
 			donor->sched_class->put_prev_task(rq, donor, donor);
 			donor->sched_class->set_next_task(rq, donor, true);
 		}
+		scx_proxy_donor_start(rq);
+		scx_proxy_resolved(rq);
 	} else {
 		rq_set_donor(rq, next);
 	}
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 285bd8422193d..c7a77cc914cd5 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -24,6 +24,11 @@
 
 DEFINE_RAW_SPINLOCK(scx_sched_lock);
 
+bool scx_allow_proxy_exec(const struct task_struct *p)
+{
+	return true;
+}
+
 /*
  * NOTE: sched_ext is in the process of growing multiple scheduler support and
  * scx_root usage is in a transitional state. Naked dereferences are safe if the
@@ -1056,6 +1061,10 @@ static void schedule_deferred_locked(struct rq *rq)
 	schedule_deferred(rq);
 }
 
+void scx_proxy_resolved(struct rq *rq)
+{
+}
+
 void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
 			u64 reenq_flags, struct rq *locked_rq)
 {
@@ -2928,6 +2937,10 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	}
 }
 
+void scx_proxy_donor_start(struct rq *rq)
+{
+}
+
 static enum scx_cpu_preempt_reason
 preempt_reason_from_class(const struct sched_class *class)
 {
diff --git a/kernel/sched/ext/ext.h b/kernel/sched/ext/ext.h
index 0b7fc46aee08c..859b9261a8a6b 100644
--- a/kernel/sched/ext/ext.h
+++ b/kernel/sched/ext/ext.h
@@ -20,6 +20,9 @@ void scx_rq_deactivate(struct rq *rq);
 int scx_check_setscheduler(struct task_struct *p, int policy);
 bool task_should_scx(int policy);
 bool scx_allow_ttwu_queue(const struct task_struct *p);
+bool scx_allow_proxy_exec(const struct task_struct *p);
+void scx_proxy_donor_start(struct rq *rq);
+void scx_proxy_resolved(struct rq *rq);
 void init_sched_ext_class(void);
 
 static inline u32 scx_cpuperf_target(s32 cpu)
@@ -54,6 +57,9 @@ static inline void scx_rq_deactivate(struct rq *rq) {}
 static inline int scx_check_setscheduler(struct task_struct *p, int policy) { return 0; }
 static inline bool task_on_scx(const struct task_struct *p) { return false; }
 static inline bool scx_allow_ttwu_queue(const struct task_struct *p) { return true; }
+static inline bool scx_allow_proxy_exec(const struct task_struct *p) { return true; }
+static inline void scx_proxy_donor_start(struct rq *rq) {}
+static inline void scx_proxy_resolved(struct rq *rq) {}
 static inline void init_sched_ext_class(void) {}
 
 #endif	/* CONFIG_SCHED_CLASS_EXT */
-- 
2.55.0

[PATCH 06/14] sched_ext: Block proxy donors across scheduler transitions

[Andrea Righi]

Proxy execution retains mutex-blocked donors on the runqueue so their
scheduling context can execute a lock owner. sched_ext cannot safely
retain such donors unless the BPF scheduler explicitly participates in
their admission and ordering.

Make sched_ext reject retained donors by default. Implement
scx_allow_proxy_exec() to force blocked EXT tasks through the regular
block path in schedule(), and fully deactivate a retained donor before
sched_setscheduler(), PI de-boosting, or global activation moves it into
the EXT class.

This is a preparatory change to support proxy execution with sched_ext.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext/ext.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index c7a77cc914cd5..bd9a8c3a269af 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -26,7 +26,15 @@ DEFINE_RAW_SPINLOCK(scx_sched_lock);
 
 bool scx_allow_proxy_exec(const struct task_struct *p)
 {
-	return true;
+	return p->sched_class != &ext_sched_class;
+}
+
+static void prepare_switch_scx(struct rq *rq, struct task_struct *p)
+{
+	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_rq_held(rq);
+
+	sched_proxy_block_task(task_rq(p), p);
 }
 
 /*
@@ -4731,6 +4739,7 @@ DEFINE_SCHED_CLASS(ext) = {
 
 	.task_tick		= task_tick_scx,
 
+	.prepare_switch		= prepare_switch_scx,
 	.switching_to		= switching_to_scx,
 	.switched_from		= switched_from_scx,
 	.switched_to		= switched_to_scx,
-- 
2.55.0

[PATCH 07/14] sched_ext: Fix ops.running/stopping() pairing for proxy-exec donors

[Andrea Righi]

With proxy execution, pick_next_task() can select a blocked task as the
scheduling context before find_proxy_task() resolves the execution
context.

From the BPF scheduler perspective, that donor is running while its
scheduling context drives the lock owner; ops.tick() and other
accounting must therefore remain enclosed by a matching
ops.running()/ops.stopping() session.

In this scenario, the session boundaries do not always match physical
task switches. Keep the "running" session open when the same donor
continues on the same CPU. When proxy execution migrates a donor's
scheduling context to another CPU, end its running session on the source
CPU and start a new session on the destination CPU only after proxy
resolution succeeds. This prevents a failed resolution from exposing a
provisional ops.running() event.

Keep track of the "running" session introducing a new
SCX_TASK_RUN_TRACKED flag.

This is a preparatory change for enabling proxy execution together with
sched_ext. The explicit running-state tracking is also required by later
donor-based accounting: it prevents an EXT owner executing for a non-EXT
donor from being treated as the active EXT scheduling context when it is
dequeued.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/sched/ext.h   |  1 +
 kernel/sched/ext/ext.c      | 62 ++++++++++++++++++++++++++++---------
 kernel/sched/ext/internal.h |  6 ++++
 3 files changed, 55 insertions(+), 14 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 78b2f289cb986..92f21b0e2ab9e 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -102,6 +102,7 @@ enum scx_ent_flags {
 	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
+	SCX_TASK_RUN_TRACKED	= 1 << 6, /* task is in an ops.running()/stopping() session */
 
 	/*
 	 * Bits 8 to 10 are used to carry task state:
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index bd9a8c3a269af..d9f9d039459f6 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -2183,10 +2183,10 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int core_deq_
 	ops_dequeue(rq, p, deq_flags);
 
 	/*
-	 * A currently running task which is going off @rq first gets dequeued
-	 * and then stops running. As we want running <-> stopping transitions
-	 * to be contained within runnable <-> quiescent transitions, trigger
-	 * ->stopping() early here instead of in put_prev_task_scx().
+	 * A current scheduling context which is going off @rq first gets
+	 * dequeued and then stops running. As we want running <-> stopping
+	 * transitions to be contained within runnable <-> quiescent transitions,
+	 * trigger ->stopping() early here instead of in put_prev_task_scx().
 	 *
 	 * @p may go through multiple stopping <-> running transitions between
 	 * here and put_prev_task_scx() if task attribute changes occur while
@@ -2194,9 +2194,12 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int core_deq_
 	 * information meaningful to the BPF scheduler and can be suppressed by
 	 * skipping the callbacks if the task is !QUEUED.
 	 */
-	if (SCX_HAS_OP(sch, stopping) && task_current(rq, p)) {
-		update_curr_scx(rq);
-		SCX_CALL_OP_TASK(sch, stopping, rq, p, false);
+	if (task_current_donor(rq, p) && (p->scx.flags & SCX_TASK_RUN_TRACKED)) {
+		if (SCX_HAS_OP(sch, stopping)) {
+			update_curr_scx(rq);
+			SCX_CALL_OP_TASK(sch, stopping, rq, p, false);
+		}
+		p->scx.flags &= ~SCX_TASK_RUN_TRACKED;
 	}
 
 	if (SCX_HAS_OP(sch, quiescent) && !task_on_rq_migrating(p))
@@ -2884,10 +2887,21 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 	return true;
 }
 
-static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
+static void scx_start_task_running(struct rq *rq, struct task_struct *p)
 {
 	struct scx_sched *sch = scx_task_sched(p);
 
+	if (p->scx.flags & SCX_TASK_RUN_TRACKED)
+		return;
+
+	if (SCX_HAS_OP(sch, running))
+		SCX_CALL_OP_TASK(sch, running, rq, p);
+
+	p->scx.flags |= SCX_TASK_RUN_TRACKED;
+}
+
+static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
+{
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		/*
 		 * Core-sched might decide to execute @p before it is
@@ -2899,9 +2913,14 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 
 	p->se.exec_start = rq_clock_task(rq);
 
-	/* see dequeue_task_scx() on why we skip when !QUEUED */
-	if (SCX_HAS_OP(sch, running) && (p->scx.flags & SCX_TASK_QUEUED))
-		SCX_CALL_OP_TASK(sch, running, rq, p);
+	/*
+	 * See dequeue_task_scx() for why we skip when !QUEUED. On a normal
+	 * scheduling transition, defer starting a blocked donor's session until
+	 * proxy resolution succeeds. A restore follows an already resolved
+	 * scheduling context and can start the session immediately.
+	 */
+	if ((p->scx.flags & SCX_TASK_QUEUED) && (!p->is_blocked || !first))
+		scx_start_task_running(rq, p);
 
 	clr_task_runnable(p, true);
 
@@ -2947,6 +2966,13 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 
 void scx_proxy_donor_start(struct rq *rq)
 {
+	struct task_struct *donor = rq->donor;
+
+	lockdep_assert_rq_held(rq);
+
+	if (donor->sched_class == &ext_sched_class &&
+	    (donor->scx.flags & SCX_TASK_QUEUED))
+		scx_start_task_running(rq, donor);
 }
 
 static enum scx_cpu_preempt_reason
@@ -3010,9 +3036,17 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 
 	update_curr_scx(rq);
 
-	/* see dequeue_task_scx() on why we skip when !QUEUED */
-	if (SCX_HAS_OP(sch, stopping) && (p->scx.flags & SCX_TASK_QUEUED))
-		SCX_CALL_OP_TASK(sch, stopping, rq, p, true);
+	/*
+	 * Preserve the running session when proxy execution refreshes the same
+	 * donor around an execution-context switch on this rq.
+	 */
+	if (next != p && (p->scx.flags & SCX_TASK_QUEUED) &&
+	    (p->scx.flags & SCX_TASK_RUN_TRACKED)) {
+		if (SCX_HAS_OP(sch, stopping))
+			SCX_CALL_OP_TASK(sch, stopping, rq, p, true);
+
+		p->scx.flags &= ~SCX_TASK_RUN_TRACKED;
+	}
 
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		set_task_runnable(rq, p);
diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index a9a853a71061c..351ffd1e02fb8 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -447,6 +447,12 @@ struct sched_ext_ops {
 	 * Therefore, always use scx_bpf_task_cpu(@p) to determine the
 	 * target CPU the task is going to use.
 	 *
+	 * Under proxy execution, the BPF scheduler continues to observe the
+	 * donor as the current scheduling context. A blocked donor enters a
+	 * ->running()/->stopping() session while its scheduling context drives
+	 * the lock owner. The lock owner executing on its behalf is intentionally
+	 * not reported through these callbacks.
+	 *
 	 * See ->runnable() for explanation on the task state notifiers.
 	 */
 	void (*running)(struct task_struct *p);
-- 
2.55.0

[PATCH 08/14] sched_ext: Generalize the reject DSQ reenqueue path

[Andrea Righi]

The reject DSQ is currently specific to sub-scheduler cap failures. Its
storage and initialization depend on CONFIG_EXT_SUB_SCHED, and the drain
path assumes every rejected task was rejected for SCX_TASK_REENQ_CAP.

Other transient placement failures need the same ability to park a task
on its source rq and return it to the owning BPF scheduler. Make the
reject DSQ unconditional, move its drain path into the core sched_ext
implementation, and record the reenqueue reason on each parked task.

This is a preparatory change to support proxy execution with sched_ext.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/sched/ext.h |  1 +
 kernel/sched/ext/ext.c    | 57 ++++++++++++++++++++++++++++++++++++---
 kernel/sched/ext/sub.c    | 47 ++------------------------------
 kernel/sched/ext/sub.h    |  2 --
 kernel/sched/sched.h      |  4 +--
 5 files changed, 57 insertions(+), 54 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 92f21b0e2ab9e..91b255fdd16bc 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -197,6 +197,7 @@ struct sched_ext_entity {
 	struct rb_node		dsq_priq;	/* p->scx.dsq_vtime order */
 	u32			dsq_seq;
 	u32			dsq_flags;	/* protected by DSQ lock */
+	u32			reject_reason;	/* protected by rq lock */
 	u32			flags;		/* protected by rq lock */
 	u32			weight;
 	s32			sticky_cpu;
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index d9f9d039459f6..2fcd303b44894 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1512,11 +1512,10 @@ static void scx_dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
 				 struct scx_dispatch_q *dsq, struct task_struct *p,
 				 u64 enq_flags)
 {
-	bool is_rq_owned = false;
+	bool is_rq_owned = dsq_is_rq_owned(dsq);
 
 	if (dsq->id == SCX_DSQ_LOCAL) {
 		dsq = scx_local_or_reject_dsq(sch, rq, p, &enq_flags);
-		is_rq_owned = true;
 	}
 
 	WARN_ON_ONCE(p->scx.dsq || !list_empty(&p->scx.dsq_list.node));
@@ -1716,6 +1715,9 @@ void scx_dispatch_dequeue(struct rq *rq, struct task_struct *p)
 	}
 	p->scx.dsq = NULL;
 
+	if (dsq->id == SCX_DSQ_REJECT)
+		p->scx.reject_reason = SCX_TASK_REENQ_NONE;
+
 	if (!is_rq_owned)
 		raw_spin_unlock(&dsq->lock);
 }
@@ -4379,6 +4381,55 @@ static void process_deferred_reenq_users(struct rq *rq)
 	}
 }
 
+/*
+ * Drain @rq->scx.reject_dsq and reenqueue each task so that its owning BPF
+ * scheduler chooses placement again.
+ *
+ * A task can be re-rejected repeatedly, and there's no repeat limit here. The
+ * private list below prevents a task from being revisited in the same round.
+ */
+static void scx_reenq_reject(struct rq *rq)
+{
+	LIST_HEAD(tasks);
+	struct task_struct *p, *n;
+
+	lockdep_assert_rq_held(rq);
+
+	if (list_empty(&rq->scx.reject_dsq.list))
+		return;
+
+	/*
+	 * Move tasks to a private list so a task re-rejected by
+	 * scx_do_enqueue_task() below isn't revisited this round.
+	 */
+	list_for_each_entry_safe(p, n, &rq->scx.reject_dsq.list, scx.dsq_list.node) {
+		u32 reason = p->scx.reject_reason;
+
+		/* migration_pending tasks should have bypassed to local DSQ */
+		if (WARN_ON_ONCE(p->migration_pending))
+			continue;
+		if (WARN_ON_ONCE(!reason))
+			continue;
+
+		scx_dispatch_dequeue(rq, p);
+		p->scx.reject_reason = SCX_TASK_REENQ_NONE;
+
+		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
+			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
+		p->scx.flags |= reason;
+
+		list_add_tail(&p->scx.dsq_list.node, &tasks);
+	}
+
+	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
+		list_del_init(&p->scx.dsq_list.node);
+
+		scx_do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);
+
+		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
+	}
+}
+
 static void run_deferred(struct rq *rq)
 {
 	process_ddsp_deferred_locals(rq);
@@ -8382,9 +8433,7 @@ void __init init_sched_ext_class(void)
 
 		/* local_dsq's sch will be set during scx_root_enable() */
 		BUG_ON(init_dsq(&rq->scx.local_dsq, SCX_DSQ_LOCAL, NULL));
-#ifdef CONFIG_EXT_SUB_SCHED
 		BUG_ON(init_dsq(&rq->scx.reject_dsq, SCX_DSQ_REJECT, NULL));
-#endif
 
 		INIT_LIST_HEAD(&rq->scx.runnable_list);
 		INIT_LIST_HEAD(&rq->scx.ddsp_deferred_locals);
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 765b516e06c95..f4ce5b74b7bfc 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -284,6 +284,8 @@ struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq
 
 	p->scx.reenq_reason_caps = missing;
 	p->scx.reenq_reason_cid = cid;
+	WARN_ON_ONCE(p->scx.reject_reason);
+	p->scx.reject_reason = SCX_TASK_REENQ_CAP;
 
 	/*
 	 * Only local DSQ can honor IMMED and dsq_inc_nr() WARNs on IMMED into
@@ -315,51 +317,6 @@ bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p)
 	return true;
 }
 
-/*
- * Drain @rq->scx.reject_dsq, reenqueueing each task so the BPF re-decides
- * from p->scx.reenq_reason_*.
- *
- * A task can be re-rejected repeatedly, and there's no repeat limit here.
- * Rejection can't happen for root, and sub-scheds can be safely ejected after
- * triggering the stall watchdog.
- */
-void scx_reenq_reject(struct rq *rq)
-{
-	LIST_HEAD(tasks);
-	struct task_struct *p, *n;
-
-	lockdep_assert_rq_held(rq);
-
-	if (!scx_has_subs() || list_empty(&rq->scx.reject_dsq.list))
-		return;
-
-	/*
-	 * Move to a private list so a task re-rejected by the
-	 * scx_do_enqueue_task() below isn't revisited this round.
-	 */
-	list_for_each_entry_safe(p, n, &rq->scx.reject_dsq.list, scx.dsq_list.node) {
-		/* migration_pending tasks should have bypassed to local DSQ */
-		if (WARN_ON_ONCE(p->migration_pending))
-			continue;
-
-		scx_dispatch_dequeue(rq, p);
-
-		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
-			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
-		p->scx.flags |= SCX_TASK_REENQ_CAP;
-
-		list_add_tail(&p->scx.dsq_list.node, &tasks);
-	}
-
-	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
-		list_del_init(&p->scx.dsq_list.node);
-
-		scx_do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);
-
-		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
-	}
-}
-
 /* record a caps change, see struct scx_caps_updated */
 static void caps_updated_record(struct scx_pshard *ps, const struct scx_cmask *cids, u64 caps,
 				struct list_head *to_deliver)
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index 625d7ce334aa8..08d46b92633a6 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -37,7 +37,6 @@ void scx_discard_stale_ecaps_syncs(void);
 struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq *rq,
 					       struct task_struct *p, u64 *enq_flags);
 bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p);
-void scx_reenq_reject(struct rq *rq);
 
 static inline const char *sch_cgrp_path(struct scx_sched *sch)
 {
@@ -72,7 +71,6 @@ static inline void scx_discard_ecaps_to_sync(s32 cpu, struct scx_sched_pcpu *pcp
 static inline void scx_discard_stale_ecaps_syncs(void) {}
 static inline struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq *rq, struct task_struct *p, u64 *enq_flags) { return &rq->scx.local_dsq; }
 static inline bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p) { return false; }
-static inline void scx_reenq_reject(struct rq *rq) {}
 static inline void scx_dec_has_subs(struct scx_sched *sch) {}
 
 #endif	/* CONFIG_EXT_SUB_SCHED */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ca436a8174d30..00a7f864ac23d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -796,9 +796,7 @@ enum scx_rq_flags {
 
 struct scx_rq {
 	struct scx_dispatch_q	local_dsq;
-#ifdef CONFIG_EXT_SUB_SCHED
-	struct scx_dispatch_q	reject_dsq;		/* staging for cap-rejected tasks */
-#endif
+	struct scx_dispatch_q	reject_dsq;		/* staging for rejected tasks */
 	struct list_head	runnable_list;		/* runnable tasks on this rq */
 	struct list_head	ddsp_deferred_locals;	/* deferred ddsps from enq */
 	unsigned long		ops_qseq;
-- 
2.55.0

[PATCH 09/14] sched_ext: Handle proxy-exec races in remote DSQ transfers

[Andrea Righi]

Without proxy execution, the DSQ lock and holding_cpu handshake ensure
that a task cannot be dequeued or start running during an rq-lock
handoff without clearing holding_cpu.

Proxy execution is an exception: a task can start physically executing
as a lock owner while its scheduling context remains on a DSQ; its
on-CPU or migration-disabled state can therefore change without clearing
holding_cpu.

Recheck these states after acquiring the source rq lock. If the transfer
can no longer proceed, park the task on the source rq's reject DSQ and
re-enqueue it through its owning scheduler. This preserves the BPF
scheduler's placement policy and keeps descendant tasks within their
sub-scheduler's cap grants.

Implement the scx_proxy_resolved() hook to drain the parked tasks once
proxy resolution has settled and the outgoing owner has switched out.

Without this change and proxy execution enabled, stress-ng --pipeherd
can trigger this race and migrate an active execution context, leading
to sleeping-while-atomic warnings and subsequent lockdep corruption.

This is a preparatory change to support proxy execution with sched_ext.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/sched/ext.h |   5 ++
 kernel/sched/ext/ext.c    | 154 +++++++++++++++++++++++++++++++++-----
 kernel/sched/sched.h      |   1 +
 3 files changed, 140 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 91b255fdd16bc..4d16c225e9af1 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -135,6 +135,9 @@ enum scx_ent_flags {
 	 * IMMED	reenqueued due to failed ENQ_IMMED
 	 * PREEMPTED	preempted while running
 	 * CAP		sub-sched cap miss, see p->scx.reenq_reason_*
+	 * MIGRATION_DISABLED
+	 *		migration-disabled during a remote DSQ transfer
+	 * PROXY	physically executing or donating during a remote DSQ transfer
 	 */
 	SCX_TASK_REENQ_REASON_SHIFT = 12,
 	SCX_TASK_REENQ_REASON_BITS = 3,
@@ -145,6 +148,8 @@ enum scx_ent_flags {
 	SCX_TASK_REENQ_IMMED	= 2 << SCX_TASK_REENQ_REASON_SHIFT,
 	SCX_TASK_REENQ_PREEMPTED = 3 << SCX_TASK_REENQ_REASON_SHIFT,
 	SCX_TASK_REENQ_CAP	= 4 << SCX_TASK_REENQ_REASON_SHIFT,
+	SCX_TASK_REENQ_MIGRATION_DISABLED = 5 << SCX_TASK_REENQ_REASON_SHIFT,
+	SCX_TASK_REENQ_PROXY	= 6 << SCX_TASK_REENQ_REASON_SHIFT,
 
 	/* iteration cursor, not a task */
 	SCX_TASK_CURSOR		= 1 << 31,
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 2fcd303b44894..95aca029a6e57 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1069,8 +1069,17 @@ static void schedule_deferred_locked(struct rq *rq)
 	schedule_deferred(rq);
 }
 
+/*
+ * Proxy resolution happens before rq->curr is switched. Queue deferred work
+ * on the rq so that an outgoing proxy owner has cleared on_cpu by the time
+ * reject_dsq is drained.
+ */
 void scx_proxy_resolved(struct rq *rq)
 {
+	lockdep_assert_rq_held(rq);
+
+	if (rq->scx.flags & SCX_RQ_PROXY_REENQ)
+		schedule_deferred_locked(rq);
 }
 
 void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
@@ -1449,9 +1458,15 @@ static void rq_owned_post_enq(struct scx_sched *sch, struct rq *rq,
 {
 	call_task_dequeue(sch, rq, p, 0);
 
-	/* rejected: kick the deferred reenq, skip wakeup/preemption */
+	/*
+	 * Proxy-active tasks must remain parked until proxy resolution. Other
+	 * rejects can be reenqueued immediately.
+	 */
 	if (unlikely(dsq->id == SCX_DSQ_REJECT)) {
-		schedule_deferred_locked(rq);
+		if (p->scx.reject_reason == SCX_TASK_REENQ_PROXY)
+			rq->scx.flags |= SCX_RQ_PROXY_REENQ;
+		else
+			schedule_deferred_locked(rq);
 		return;
 	}
 
@@ -2350,8 +2365,10 @@ static void move_remote_task_to_local_dsq(struct scx_sched *sch,
  * - The BPF scheduler is bypassed while the rq is offline and we can always say
  *   no to the BPF scheduler initiated migrations while offline.
  *
- * The caller must ensure that @p and @rq are on different CPUs.
- * If enforce == true, caller must hold @p's rq lock.
+ * The caller must ensure that @p and @rq are on different CPUs. If @enforce is
+ * true, report violations attributable to BPF-directed migrations. The caller
+ * must hold @p's rq lock to avoid reporting a transient race as a scheduler
+ * error.
  */
 static bool task_can_run_on_remote_rq(struct scx_sched *sch,
 				      struct task_struct *p, struct rq *rq,
@@ -2359,11 +2376,6 @@ static bool task_can_run_on_remote_rq(struct scx_sched *sch,
 {
 	s32 cpu = cpu_of(rq);
 
-	/*
-	 * To prevent races with @p still running on its old CPU while switching
-	 * out, make sure we're holding @p's rq lock so as not to risk
-	 * erroneously killing the BPF scheduler.
-	 */
 	if (enforce)
 		lockdep_assert_rq_held(task_rq(p));
 
@@ -2410,6 +2422,60 @@ static bool task_can_run_on_remote_rq(struct scx_sched *sch,
 	return true;
 }
 
+/*
+ * Proxy execution can change @p's execution and migration-disabled state
+ * without touching its DSQ entry or clearing holding_cpu. Check those states
+ * with @p's rq locked. Without proxy execution, the holding_cpu handshake is
+ * sufficient and this must not affect the existing migration path.
+ */
+static u32 task_move_reject_reason(struct task_struct *p)
+{
+	struct rq *src_rq = task_rq(p);
+
+	lockdep_assert_rq_held(src_rq);
+
+	if (!sched_proxy_exec())
+		return SCX_TASK_REENQ_NONE;
+
+	/* @p may be rq->curr under another task's scheduling context. */
+	if (task_on_cpu(src_rq, p))
+		return SCX_TASK_REENQ_PROXY;
+
+	/*
+	 * Reject only BPF-directed migration. proxy_migrate_task() may still
+	 * move a blocked donor's scheduling context to its lock owner's CPU.
+	 */
+	if (is_migration_disabled(p))
+		return SCX_TASK_REENQ_MIGRATION_DISABLED;
+
+	/* Don't move an active scheduling context off its source rq. */
+	if (task_current_donor(src_rq, p))
+		return SCX_TASK_REENQ_PROXY;
+
+	return SCX_TASK_REENQ_NONE;
+}
+
+/*
+ * Park a task whose remote transfer raced with proxy execution. Reenqueueing
+ * from the source rq makes the task's owning scheduler choose its placement
+ * again and preserves sub-scheduler containment.
+ */
+static void scx_reject_task(struct scx_sched *sch, struct rq *rq,
+			    struct task_struct *p, u64 enq_flags, u32 reason)
+{
+	lockdep_assert_rq_held(rq);
+	WARN_ON_ONCE(reason != SCX_TASK_REENQ_MIGRATION_DISABLED &&
+		     reason != SCX_TASK_REENQ_PROXY);
+	WARN_ON_ONCE(p->scx.reject_reason);
+
+	p->scx.holding_cpu = -1;
+	p->scx.reject_reason = reason;
+	p->scx.flags &= ~SCX_TASK_IMMED;
+	enq_flags &= ~(SCX_ENQ_IMMED | SCX_ENQ_PREEMPT);
+
+	scx_dispatch_enqueue(sch, rq, &rq->scx.reject_dsq, p, enq_flags);
+}
+
 /**
  * unlink_dsq_and_switch_rq_lock() - Unlink task and switch to its rq lock
  * @p: target task
@@ -2467,6 +2533,23 @@ static bool consume_remote_task(struct scx_sched *sch, struct rq *this_rq,
 				struct scx_dispatch_q *dsq, struct rq *src_rq)
 {
 	if (unlink_dsq_and_switch_rq_lock(p, dsq, this_rq, src_rq)) {
+		u32 reject_reason = task_move_reject_reason(p);
+
+		/*
+		 * Proxy execution may have changed @p's running or
+		 * migration-disabled state while switching rq locks without
+		 * clearing holding_cpu. Park it on the source rq and let its
+		 * owning scheduler choose its placement again.
+		 */
+		if (unlikely(reject_reason)) {
+			p->scx.dsq = NULL;
+			scx_reject_task(sch, src_rq, p,
+					enq_flags | SCX_ENQ_CLEAR_OPSS,
+					reject_reason);
+			switch_rq_lock(src_rq, this_rq);
+			return false;
+		}
+
 		move_remote_task_to_local_dsq(sch, p, enq_flags, src_rq, this_rq);
 		return true;
 	} else {
@@ -2497,6 +2580,7 @@ static struct rq *move_task_between_dsqs(struct scx_sched *sch,
 					 struct scx_dispatch_q *dst_dsq)
 {
 	struct rq *src_rq = task_rq(p), *dst_rq;
+	u32 reject_reason;
 
 	BUG_ON(src_dsq->id == SCX_DSQ_LOCAL);
 	lockdep_assert_held(&src_dsq->lock);
@@ -2504,6 +2588,15 @@ static struct rq *move_task_between_dsqs(struct scx_sched *sch,
 
 	if (dst_dsq->id == SCX_DSQ_LOCAL) {
 		dst_rq = container_of(dst_dsq, struct rq, scx.local_dsq);
+		reject_reason = src_rq != dst_rq ?
+			task_move_reject_reason(p) : SCX_TASK_REENQ_NONE;
+		if (unlikely(reject_reason)) {
+			dispatch_dequeue_locked(p, src_dsq);
+			raw_spin_unlock(&src_dsq->lock);
+			scx_reject_task(sch, src_rq, p, enq_flags,
+					reject_reason);
+			return src_rq;
+		}
 		if (src_rq != dst_rq &&
 		    unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
 			dst_dsq = find_global_dsq(sch, task_cpu(p));
@@ -2659,6 +2752,11 @@ static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
 	if (likely(p->scx.holding_cpu == raw_smp_processor_id()) &&
 	    !WARN_ON_ONCE(src_rq != task_rq(p))) {
 		bool fallback = false;
+		u32 reject_reason;
+
+		reject_reason = src_rq != dst_rq ?
+			task_move_reject_reason(p) : SCX_TASK_REENQ_NONE;
+
 		/*
 		 * If @p is staying on the same rq, there's no need to go
 		 * through the full deactivate/activate cycle. Optimize by
@@ -2668,9 +2766,14 @@ static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
 			p->scx.holding_cpu = -1;
 			scx_dispatch_enqueue(sch, dst_rq, &dst_rq->scx.local_dsq, p,
 					     enq_flags);
-		} else if (unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
-			p->scx.holding_cpu = -1;
+		} else if (unlikely(reject_reason)) {
 			fallback = true;
+			scx_reject_task(sch, src_rq, p, enq_flags,
+					reject_reason);
+		} else if (unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq,
+							       true))) {
+			fallback = true;
+			p->scx.holding_cpu = -1;
 			scx_dispatch_enqueue(sch, src_rq, find_global_dsq(sch, task_cpu(p)),
 					     p, enq_flags | SCX_ENQ_GDSQ_FALLBACK);
 		} else {
@@ -4382,35 +4485,41 @@ static void process_deferred_reenq_users(struct rq *rq)
 }
 
 /*
- * Drain @rq->scx.reject_dsq and reenqueue each task so that its owning BPF
- * scheduler chooses placement again.
- *
- * A task can be re-rejected repeatedly, and there's no repeat limit here. The
- * private list below prevents a task from being revisited in the same round.
+ * Drain ready tasks from @rq->scx.reject_dsq and reenqueue them so that their
+ * owning BPF schedulers choose placement again. Proxy-active tasks remain
+ * parked until proxy resolution schedules another drain after switch-out.
  */
 static void scx_reenq_reject(struct rq *rq)
 {
 	LIST_HEAD(tasks);
 	struct task_struct *p, *n;
+	bool proxy_pending = false;
 
 	lockdep_assert_rq_held(rq);
 
-	if (list_empty(&rq->scx.reject_dsq.list))
+	if (list_empty(&rq->scx.reject_dsq.list)) {
+		rq->scx.flags &= ~SCX_RQ_PROXY_REENQ;
 		return;
+	}
 
 	/*
-	 * Move tasks to a private list so a task re-rejected by
+	 * Move ready tasks to a private list so a task re-rejected by
 	 * scx_do_enqueue_task() below isn't revisited this round.
 	 */
 	list_for_each_entry_safe(p, n, &rq->scx.reject_dsq.list, scx.dsq_list.node) {
 		u32 reason = p->scx.reject_reason;
 
 		/* migration_pending tasks should have bypassed to local DSQ */
-		if (WARN_ON_ONCE(p->migration_pending))
-			continue;
+		WARN_ON_ONCE(p->migration_pending);
 		if (WARN_ON_ONCE(!reason))
 			continue;
 
+		if (reason == SCX_TASK_REENQ_PROXY &&
+		    (task_on_cpu(rq, p) || task_current_donor(rq, p))) {
+			proxy_pending = true;
+			continue;
+		}
+
 		scx_dispatch_dequeue(rq, p);
 		p->scx.reject_reason = SCX_TASK_REENQ_NONE;
 
@@ -4421,6 +4530,11 @@ static void scx_reenq_reject(struct rq *rq)
 		list_add_tail(&p->scx.dsq_list.node, &tasks);
 	}
 
+	if (proxy_pending)
+		rq->scx.flags |= SCX_RQ_PROXY_REENQ;
+	else
+		rq->scx.flags &= ~SCX_RQ_PROXY_REENQ;
+
 	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
 		list_del_init(&p->scx.dsq_list.node);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 00a7f864ac23d..fdcd723c29a3c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -789,6 +789,7 @@ enum scx_rq_flags {
 	SCX_RQ_BAL_CB_PENDING	= 1 << 6, /* must queue a cb after dispatching */
 	SCX_RQ_SUB_IDLE_RENOTIFY	= 1 << 7, /* sub-scheds are owed update_idle() */
 	SCX_RQ_ROOT_IDLE_RENOTIFY	= 1 << 8, /* the root is owed update_idle() */
+	SCX_RQ_PROXY_REENQ	= 1 << 9, /* proxy-rejected tasks need reenqueue */
 
 	SCX_RQ_IN_WAKEUP	= 1 << 16,
 	SCX_RQ_IN_BALANCE	= 1 << 17,
-- 
2.55.0

[PATCH 10/14] sched_ext: Split curr|donor references properly

[Andrea Righi]

With proxy execution, the task selected by the scheduler and the task
physically executing can differ. A blocked mutex waiter donates its
scheduling context to the lock owner:

  D -----------------> M -------------> O ----------------> T
  [donor] blocked on [mutex] owned by [owner] preempted by [task]
     \_________________________________^
          donates scheduling context

where:

  D = blocked donor
  M = mutex
  O = mutex owner
  T = competing runnable task

During a proxy execution switch, D supplies the scheduling class,
priority, and runtime budget, while O supplies the execution context: O
is the task whose code physically executes. T is a competing runnable
task which may preempt the D/O proxy execution.

Consider FAIR and EXT tasks with sched_ext running in partial mode. FAIR
can be replaced with a higher scheduling class such as RT or deadline
without changing the class interaction described here. The possible
combinations are:

  1. D is EXT, O is EXT, T is EXT

     D can interrupt T according to BPF scheduling policy. O executes
     with D's EXT priority and runtime budget, while T waits in EXT.

  2. D is EXT, O is EXT, T is FAIR

     D is visible to the BPF scheduler, but cannot preempt T because
     EXT is below FAIR. Once T stops, BPF can dispatch D and O executes
     with D's EXT priority and runtime budget. If T becomes runnable
     again, it preempts the D/O proxy execution.

  3. D is EXT, O is FAIR, T is EXT

     This cannot represent T preempting O because EXT is below FAIR.

  4. D is EXT, O is FAIR, T is FAIR

     D cannot boost O above T because EXT is below FAIR. O and T
     continue competing under FAIR. Once O releases M, D wakes and
     resumes normal EXT scheduling.

  5. D is FAIR, O is EXT, T is EXT

     D preempts T as the higher-class scheduling context. O executes
     with D's FAIR priority and runtime budget, while T waits in EXT.
     D is not visible to the BPF scheduler.

  6. D is FAIR, O is EXT, T is FAIR

     D competes with T according to its FAIR deadline. When D is
     selected, O executes with D's FAIR priority and runtime budget.
     D is not visible to the BPF scheduler.

  7. D is FAIR, O is FAIR, T is EXT

     This cannot represent T preempting O because EXT is below FAIR.

  8. D is FAIR, O is FAIR, T is FAIR

     O, T, and D all have FAIR scheduling contexts. D remains runnable
     as a blocked proxy donor. When CFS selects D, O executes using D's
     FAIR scheduling context. When CFS selects O, O executes using its
     own FAIR context, and when CFS selects T, T executes normally. D
     is not visible to the BPF scheduler.

Thus, sched_ext policy and accounting must generally use rq->donor, the
scheduler-selected task which supplies the scheduling context, rather
than rq->curr, the task whose code physically executes. Without proxy
execution they are the same task.

On nohz_full CPUs, a blocked proxy donor must retain the scheduler tick
even when it has an infinite slice. Otherwise, a full dynticks CPU could
stop the tick while rq->curr and rq->donor differ, violating assumptions
made by the remote NOHZ tick path.

This is a conservative compromise that keeps the change local to
sched_ext, at the cost of a periodic tick while a blocked proxy donor is
selected. Allowing blocked proxy donors to run tickless would require
making the core scheduler's remote tick handling aware that rq->curr and
rq->donor can differ.

Moreover, extend scx_dump_state() to report both contexts. Each CPU
record now includes a donor= line. If an EXT donor differs from
rq->curr, also emit its detailed task record. The existing '*' marker
continues to identify rq->curr, while the donor= line identifies the
otherwise unmarked donor record.

Note that at this point in the series, CONFIG_SCHED_PROXY_EXEC still
depends on !CONFIG_SCHED_CLASS_EXT, so proxy execution and sched_ext
cannot be enabled together. The scheduling changes are therefore
preparatory. A later patch removes this restriction.

Co-developed-by: John Stultz <jstultz@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 Documentation/scheduler/sched-ext.rst |   6 ++
 kernel/sched/ext/ext.c                | 115 +++++++++++++++++---------
 kernel/sched/ext/sub.h                |   8 +-
 3 files changed, 86 insertions(+), 43 deletions(-)

diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
index 2771ea4cc14af..4d8bcbdacb9fc 100644
--- a/Documentation/scheduler/sched-ext.rst
+++ b/Documentation/scheduler/sched-ext.rst
@@ -487,6 +487,12 @@ and edge cases, to name a few examples:
   class, in which case it will exit the tick-dispatch loop even though it is runnable
   and has a non-zero slice.
 
+* Under proxy execution, sched_ext continues to observe the donor as the current
+  scheduling context. A blocked donor does not enter an ``ops.running()`` /
+  ``ops.stopping()`` session because it does not execute itself, and the lock
+  owner executing on its behalf is intentionally not reported through these
+  callbacks.
+
 See the "Scheduling Cycle" section for a more detailed description of how
 a freshly woken up task gets on a CPU.
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 95aca029a6e57..c6720e5c78dad 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1327,20 +1327,27 @@ static void apply_task_slice_oob(struct rq *rq, struct task_struct *p)
 
 static void update_curr_scx(struct rq *rq)
 {
-	struct task_struct *curr = rq->curr;
+	struct task_struct *donor;
 	s64 delta_exec;
 
+	/*
+	 * update_curr_scx() is selected through rq->donor->sched_class, not
+	 * rq->curr->sched_class, so @donor is always an EXT task here. If an EXT
+	 * owner executes for a FAIR donor, FAIR's update_curr() runs instead.
+	 */
+	donor = rq->donor;
+
 	/* apply even on 0 delta_exec, callers may still act on the slice */
-	apply_task_slice_oob(rq, curr);
+	apply_task_slice_oob(rq, donor);
 
 	delta_exec = update_curr_common(rq);
 	if (unlikely(delta_exec <= 0))
 		return;
 
-	if (curr->scx.slice != SCX_SLICE_INF) {
-		curr->scx.slice -= min_t(u64, curr->scx.slice, delta_exec);
-		if (!curr->scx.slice)
-			touch_core_sched(rq, curr);
+	if (donor->scx.slice != SCX_SLICE_INF) {
+		donor->scx.slice -= min_t(u64, donor->scx.slice, delta_exec);
+		if (!donor->scx.slice)
+			touch_core_sched(rq, donor);
 	}
 
 	dl_server_update(&rq->ext_server, delta_exec);
@@ -1516,9 +1523,9 @@ static void rq_owned_post_enq(struct scx_sched *sch, struct rq *rq,
 	if (rq->scx.flags & SCX_RQ_IN_BALANCE)
 		return;
 
-	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
-	    rq->curr->sched_class == &ext_sched_class) {
-		set_task_slice(rq->curr, 0);
+	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->donor &&
+	    rq->donor->sched_class == &ext_sched_class) {
+		set_task_slice(rq->donor, 0);
 		resched_curr(rq);
 	}
 }
@@ -2061,13 +2068,14 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_
 		rq->scx.flags |= SCX_RQ_IN_WAKEUP;
 
 	/*
-	 * Restoring a running task will be immediately followed by
-	 * set_next_task_scx() which expects the task to not be on the BPF
+	 * Restoring the current scheduling context will be immediately followed
+	 * by set_next_task_scx() which expects the task to not be on the BPF
 	 * scheduler as tasks can only start running through local DSQs. Force
 	 * direct-dispatch into the local DSQ by setting the sticky_cpu. Mark
 	 * IGNORE_CAPS to force entry into the local DSQ.
 	 */
-	if (unlikely(enq_flags & ENQUEUE_RESTORE) && task_current(rq, p)) {
+	if (unlikely(enq_flags & ENQUEUE_RESTORE) &&
+	    task_current_donor(rq, p)) {
 		sticky_cpu = cpu_of(rq);
 		enq_flags |= SCX_ENQ_IGNORE_CAPS;
 	}
@@ -2783,7 +2791,8 @@ static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
 		}
 
 		/* if the destination CPU is idle, wake it up */
-		if (!fallback && sched_class_above(p->sched_class, dst_rq->curr->sched_class))
+		if (!fallback && sched_class_above(p->sched_class,
+						      dst_rq->donor->sched_class))
 			resched_curr(dst_rq);
 	}
 
@@ -3007,6 +3016,8 @@ static void scx_start_task_running(struct rq *rq, struct task_struct *p)
 
 static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 {
+	bool can_stop_tick;
+
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		/*
 		 * Core-sched might decide to execute @p before it is
@@ -3031,6 +3042,7 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 
 	/* apply any pending out-of-band slice request before the tick decision */
 	apply_task_slice_oob(rq, p);
+	can_stop_tick = p->scx.slice == SCX_SLICE_INF && !p->is_blocked;
 
 	/*
 	 * @p is getting newly scheduled or got kicked after someone updated its
@@ -3041,7 +3053,7 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	 * nohz. In the future, we might want to add a mechanism to update
 	 * load_avgs periodically on tick-stopped CPUs.
 	 */
-	if (p->scx.slice == SCX_SLICE_INF) {
+	if (can_stop_tick) {
 		if (!(rq->scx.flags & SCX_RQ_CAN_STOP_TICK)) {
 			/*
 			 * Bypass mode always assigns finite slices, so @p
@@ -3062,7 +3074,8 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 
 		/*
 		 * @rq still references the outgoing scheduling context. A finite
-		 * slice is sufficient by itself to require the tick.
+		 * slice or a blocked proxy donor is sufficient by itself to require
+		 * the tick.
 		 */
 		if (tick_nohz_full_cpu(cpu_of(rq)))
 			tick_nohz_dep_set_cpu(cpu_of(rq), TICK_DEP_BIT_SCHED);
@@ -3251,7 +3264,7 @@ static struct task_struct *first_local_task(struct rq *rq)
 static struct task_struct *
 do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
 {
-	struct task_struct *prev = rq->curr;
+	struct task_struct *prev = rq->donor;
 	bool keep_prev;
 	struct task_struct *p;
 
@@ -3650,9 +3663,9 @@ void scx_tick(struct rq *rq)
 	update_other_load_avgs(rq);
 }
 
-static void task_tick_scx(struct rq *rq, struct task_struct *curr, int queued)
+static void task_tick_scx(struct rq *rq, struct task_struct *donor, int queued)
 {
-	struct scx_sched *sch = scx_task_sched(curr);
+	struct scx_sched *sch = scx_task_sched(donor);
 
 	update_curr_scx(rq);
 
@@ -3661,13 +3674,13 @@ static void task_tick_scx(struct rq *rq, struct task_struct *curr, int queued)
 	 * we can't trust the slice management or ops.core_sched_before().
 	 */
 	if (scx_bypassing(sch, cpu_of(rq))) {
-		set_task_slice(curr, 0);
-		touch_core_sched(rq, curr);
+		set_task_slice(donor, 0);
+		touch_core_sched(rq, donor);
 	} else if (SCX_HAS_OP(sch, tick)) {
-		SCX_CALL_OP_TASK(sch, tick, rq, curr);
+		SCX_CALL_OP_TASK(sch, tick, rq, donor);
 	}
 
-	if (!curr->scx.slice)
+	if (!donor->scx.slice)
 		resched_curr(rq);
 }
 
@@ -4305,16 +4318,16 @@ static u32 reenq_local(struct scx_sched *sch, struct rq *rq, u64 reenq_flags)
 	}
 
 	/*
-	 * The revoke that scheduled this scan may have raced the pick: curr
+	 * The revoke that scheduled this scan may have raced the pick: donor
 	 * may be a now-capless task, either one that kept running or one
 	 * promoted off the local DSQ between the ecaps sync and this scan.
 	 * Zero the slice to evict it. The enqueue gate blocks new capless
 	 * inserts, so no later pick can slip through after the scan.
 	 */
 	if ((reenq_flags & SCX_REENQ_CAP_REVOKE) &&
-	    rq->curr->sched_class == &ext_sched_class &&
-	    scx_task_reenq_on_cap_revoke(rq, rq->curr)) {
-		set_task_slice(rq->curr, 0);
+	    rq->donor->sched_class == &ext_sched_class &&
+	    scx_task_reenq_on_cap_revoke(rq, rq->donor)) {
+		set_task_slice(rq->donor, 0);
 		resched_curr(rq);
 	}
 
@@ -4560,14 +4573,18 @@ static void run_deferred(struct rq *rq)
 #ifdef CONFIG_NO_HZ_FULL
 bool scx_can_stop_tick(struct rq *rq)
 {
-	struct task_struct *p = rq->curr;
+	struct task_struct *p = rq->donor;
 	struct scx_sched *sch = scx_task_sched(p);
 
+	/* The remote tick path assumes that proxy execution is not active. */
+	if (rq->curr != rq->donor)
+		return false;
+
 	if (p->sched_class != &ext_sched_class)
 		return true;
 
 	/*
-	 * @rq->curr may still reference an outgoing EXT task after it has been
+	 * @rq->donor may still reference an outgoing EXT task after it has been
 	 * dequeued. If no EXT tasks are accounted on @rq, ignore its stale
 	 * slice state. If another task is dispatched from a DSQ,
 	 * set_next_task_scx() will update the dependency for the incoming task.
@@ -4581,7 +4598,8 @@ bool scx_can_stop_tick(struct rq *rq)
 	/*
 	 * @rq can dispatch from different DSQs, so we can't tell whether it
 	 * needs the tick or not by looking at nr_running. Allow stopping ticks
-	 * iff the BPF scheduler indicated so. See set_next_task_scx().
+	 * iff set_next_task_scx() determined that the selected scheduling context
+	 * can run tickless.
 	 */
 	return rq->scx.flags & SCX_RQ_CAN_STOP_TICK;
 }
@@ -6775,6 +6793,9 @@ static void scx_dump_cpu(struct scx_sched *sch, struct seq_buf *s,
 	dump_line(&ns, "          curr=%s[%d] class=%ps",
 		  rq->curr->comm, rq->curr->pid,
 		  rq->curr->sched_class);
+	dump_line(&ns, "          donor=%s[%d] class=%ps",
+		  rq->donor->comm, rq->donor->pid,
+		  rq->donor->sched_class);
 	if (!cpumask_empty(pcpu->cpus_to_kick))
 		dump_line(&ns, "  cpus_to_kick   : %*pb",
 			  cpumask_pr_args(pcpu->cpus_to_kick));
@@ -6818,6 +6839,10 @@ static void scx_dump_cpu(struct scx_sched *sch, struct seq_buf *s,
 	if (rq->curr->sched_class == &ext_sched_class &&
 	    (dump_all_tasks || scx_task_on_sched(sch, rq->curr)))
 		scx_dump_task(sch, s, dctx, rq, rq->curr, '*');
+	if (rq->donor != rq->curr &&
+	    rq->donor->sched_class == &ext_sched_class &&
+	    (dump_all_tasks || scx_task_on_sched(sch, rq->donor)))
+		scx_dump_task(sch, s, dctx, rq, rq->donor, ' ');
 
 	list_for_each_entry(p, &rq->scx.runnable_list, scx.runnable_node)
 		if (dump_all_tasks || scx_task_on_sched(sch, p))
@@ -8331,7 +8356,7 @@ static bool kick_one_cpu(s32 cpu, struct scx_sched_pcpu *pcpu, struct rq *this_r
 	unsigned long flags;
 
 	raw_spin_rq_lock_irqsave(rq, flags);
-	cur_class = rq->curr->sched_class;
+	cur_class = rq->donor->sched_class;
 
 	/*
 	 * During CPU hotplug, a CPU may depend on kicking itself to make
@@ -8348,7 +8373,7 @@ static bool kick_one_cpu(s32 cpu, struct scx_sched_pcpu *pcpu, struct rq *this_r
 			if (cur_class == &ext_sched_class) {
 				if (likely(!scx_missing_caps(pcpu->sch, cpu,
 							     scx_caps_for_preempt(pcpu->sch, rq))))
-					set_task_slice(rq->curr, 0);
+					set_task_slice(rq->donor, 0);
 				else
 					__scx_add_event(pcpu->sch,
 							SCX_EV_SUB_PREEMPT_DENIED, 1);
@@ -9317,13 +9342,15 @@ __bpf_kfunc bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice,
 		return false;
 
 	/*
-	 * Directly write only when we hold the lock of the rq @p is queued or
-	 * running on. See the slice write rules above.
+	 * Directly write only when we hold the lock of the rq @p is queued on or
+	 * provides the current scheduling context for. Under proxy execution,
+	 * rq->donor owns and consumes the slice while rq->curr executes on its
+	 * behalf. See the slice write rules above.
 	 */
 	locked_rq = scx_locked_rq();
 	if (!locked_rq ||
 	    (READ_ONCE(p->scx.runnable_cpu) != cpu_of(locked_rq) &&
-	     !task_current(locked_rq, p))) {
+	     !task_current_donor(locked_rq, p))) {
 		set_task_slice_oob(sch, p, slice);
 		return true;
 	}
@@ -10160,12 +10187,17 @@ __bpf_kfunc void scx_bpf_put_cpumask(const struct cpumask *cpumask)
 }
 
 /**
- * scx_bpf_task_running - Is task currently running?
+ * scx_bpf_task_running - Is task the current scheduling context?
  * @p: task of interest
+ *
+ * Under proxy execution, this reports the donor rather than the task whose
+ * code is physically executing. The physical execution context is intentionally
+ * not exposed to the BPF scheduler, which continues to observe the donor as the
+ * running scheduling context.
  */
 __bpf_kfunc bool scx_bpf_task_running(const struct task_struct *p)
 {
-	return task_rq(p)->curr == p;
+	return rcu_access_pointer(task_rq(p)->donor) == p;
 }
 
 /**
@@ -10226,10 +10258,15 @@ __bpf_kfunc struct rq *scx_bpf_locked_rq(const struct bpf_prog_aux *aux)
 }
 
 /**
- * scx_bpf_cpu_curr - Return remote CPU's curr task
+ * scx_bpf_cpu_curr - Return remote CPU's current scheduling context
  * @cpu: CPU of interest
  * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
  *
+ * Under proxy execution, this returns the donor, which supplies the scheduling
+ * policy and runtime budget, rather than the task whose code is physically
+ * executing. The physical execution context is intentionally not exposed to
+ * the BPF scheduler.
+ *
  * Callers must hold RCU read lock (KF_RCU).
  */
 __bpf_kfunc struct task_struct *scx_bpf_cpu_curr(s32 cpu, const struct bpf_prog_aux *aux)
@@ -10245,7 +10282,7 @@ __bpf_kfunc struct task_struct *scx_bpf_cpu_curr(s32 cpu, const struct bpf_prog_
 	if (!scx_cpu_valid(sch, cpu, NULL))
 		return NULL;
 
-	return rcu_dereference(cpu_rq(cpu)->curr);
+	return rcu_dereference(cpu_rq(cpu)->donor);
 }
 
 /**
@@ -10269,7 +10306,7 @@ __bpf_kfunc struct task_struct *scx_bpf_cid_curr(s32 cid, const struct bpf_prog_
 	cpu = scx_cid_to_cpu(sch, cid);
 	if (cpu < 0)
 		return NULL;
-	return rcu_dereference(cpu_rq(cpu)->curr);
+	return rcu_dereference(cpu_rq(cpu)->donor);
 }
 
 /**
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index 08d46b92633a6..e2e3c2f52a146 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -139,14 +139,14 @@ static inline u64 scx_caps_for_task(struct task_struct *p)
 	return SCX_CAP_ENQ;
 }
 
-/* the cap @sch needs to preempt @rq's current task, 0 if none */
+/* the cap @sch needs to preempt @rq's current scheduling context, 0 if none */
 static inline u64 scx_caps_for_preempt(struct scx_sched *sch, struct rq *rq)
 {
-	struct task_struct *curr = rq->curr;
+	struct task_struct *donor = rq->donor;
 
 	/* a non-ext task can't be preempted by ext, own-subtree needs no cap */
-	if (curr->sched_class != &ext_sched_class ||
-	    scx_is_descendant(scx_task_sched(curr), sch))
+	if (donor->sched_class != &ext_sched_class ||
+	    scx_is_descendant(scx_task_sched(donor), sch))
 		return 0;
 	return SCX_CAP_PREEMPT;
 }
-- 
2.55.0

[PATCH 11/14] sched_ext: Delegate proxy donor admission to BPF schedulers

[Andrea Righi]

Proxy execution keeps a mutex-blocked donor runnable so that its
scheduling context can execute the mutex owner.

Define SCX_OPS_ENQ_BLOCKED as both an admission contract and an
ownership rule for retained donors. Schedulers without the flag block
EXT donors normally. Schedulers with the flag continue to own blocked
donors and receive them through ops.enqueue() with SCX_ENQ_BLOCKED,
allowing BPF to choose their DSQ, CPU and ordering.

Once BPF dispatches a donor, proxy execution may move its scheduling
context to the mutex owner's rq. This move is distinct from BPF
placement: wake_cpu continues to identify the donor's callback CPU,
while task_cpu() identifies the proxy CPU. When the donor wakes, the
normal wakeup path uses wake_cpu as prev_cpu when returning the task to
the BPF scheduler, which can then choose its next placement normally.

Apply the same admission contract across scheduler ownership changes: a
retained donor moving into a scheduler without SCX_OPS_ENQ_BLOCKED is
fully deactivated and remains on the mutex wait path; a donor moving
into an opted-in scheduler remains queued and is handed to that
scheduler. Tasks without an assigned scheduler retain the generic
proxy-execution fallback during the transition.

Blocked-donor enqueueing takes precedence over the exiting and
migration-disabled local-DSQ fallbacks, so that an opted-in scheduler
sees every eligible donor request.

Putting these rules together, the donor admission flow is the following:

    D ------ blocked on -----> M ------ owned by -----> O
 [donor]                    [mutex]                [owner]
    |
    v
 +--------------------------+
 | SCX_OPS_ENQ_BLOCKED set? |
 +------------+-------------+
          no  |  yes
       +------+-------+
       |              |
       v              v
 block normally   D runnable
                      |
                      v
          ops.enqueue(D, SCX_ENQ_BLOCKED)
                      |
                      v
             BPF admission policy
                      |
                  D selected
                      |
                      v
             proxy-exec resolves
                 D -> M -> O
                      |
                      v
               rq->donor = D
               rq->curr  = O
                      |
                O releases M
                      |
                      v
               D returns through
                the wakeup path
                      |
                      v
              BPF handles D as
              an unblocked task

Co-developed-by: John Stultz <jstultz@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/sched/ext.h                     |   1 +
 kernel/sched/ext/ext.c                        | 155 ++++++++++++++++--
 kernel/sched/ext/internal.h                   |  23 ++-
 kernel/sched/ext/sub.c                        |   6 +-
 tools/sched_ext/include/scx/compat.h          |   1 +
 .../sched_ext/include/scx/enum_defs.autogen.h |   1 +
 .../sched_ext/include/scx/enums.autogen.bpf.h |   3 +
 tools/sched_ext/include/scx/enums.autogen.h   |   1 +
 8 files changed, 171 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 4d16c225e9af1..c0fe88c2a9010 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -103,6 +103,7 @@ enum scx_ent_flags {
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
 	SCX_TASK_RUN_TRACKED	= 1 << 6, /* task is in an ops.running()/stopping() session */
+	SCX_TASK_ENQ_WAKEUP	= 1 << 7, /* wakeup enqueue awaiting wakeup_preempt() */
 
 	/*
 	 * Bits 8 to 10 are used to carry task state:
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index c6720e5c78dad..7bd1b801448da 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -26,15 +26,51 @@ DEFINE_RAW_SPINLOCK(scx_sched_lock);
 
 bool scx_allow_proxy_exec(const struct task_struct *p)
 {
-	return p->sched_class != &ext_sched_class;
+	struct scx_sched *sch;
+
+	if (p->sched_class != &ext_sched_class)
+		return true;
+
+	/*
+	 * scx_enabled() may change while __schedule() holds only @p's rq lock.
+	 * Once @p is associated with a scheduler, use that scheduler's policy
+	 * even while the global enable state is transitioning.
+	 */
+	sch = scx_task_sched(p);
+	return !sch || (sch->ops.flags & SCX_OPS_ENQ_BLOCKED);
 }
 
 static void prepare_switch_scx(struct rq *rq, struct task_struct *p)
 {
+	struct scx_sched *sch;
+
 	lockdep_assert_held(&p->pi_lock);
 	lockdep_assert_rq_held(rq);
 
-	sched_proxy_block_task(task_rq(p), p);
+	sch = scx_task_sched(p);
+	if (WARN_ON_ONCE(!sch))
+		return;
+
+	/* Block retained donors that the incoming scheduler cannot manage. */
+	if (!(sch->ops.flags & SCX_OPS_ENQ_BLOCKED))
+		sched_proxy_block_task(rq, p);
+}
+
+/*
+ * Called with @p's pi and rq locks held immediately before
+ * sched_change_begin(). The caller must pass DEQUEUE_NOCLOCK so the rq clock
+ * is updated only once.
+ */
+void scx_prepare_task_sched_change(struct task_struct *p, struct scx_sched *sch)
+{
+	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_rq_held(task_rq(p));
+
+	update_rq_clock(task_rq(p));
+
+	/* Block retained donors that the incoming scheduler cannot manage. */
+	if (!(sch->ops.flags & SCX_OPS_ENQ_BLOCKED))
+		sched_proxy_block_task(task_rq(p), p);
 }
 
 /*
@@ -1916,6 +1952,7 @@ void scx_do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
 	struct scx_sched *sch = scx_task_sched(p);
 	struct task_struct **ddsp_taskp;
 	struct scx_dispatch_q *dsq;
+	bool enq_blocked;
 	unsigned long qseq;
 
 	WARN_ON_ONCE(!(p->scx.flags & SCX_TASK_QUEUED));
@@ -1948,18 +1985,24 @@ void scx_do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
 	if (p->scx.ddsp_dsq_id != SCX_DSQ_INVALID)
 		goto direct;
 
-	/* see %SCX_OPS_ENQ_EXITING */
-	if (!(sch->ops.flags & SCX_OPS_ENQ_EXITING) &&
-	    unlikely(p->flags & PF_EXITING)) {
-		__scx_add_event(sch, SCX_EV_ENQ_SKIP_EXITING, 1);
-		goto local;
-	}
+	enq_blocked = (sch->ops.flags & SCX_OPS_ENQ_BLOCKED) &&
+		      p->is_blocked && !(enq_flags & SCX_ENQ_WAKEUP);
+	if (enq_blocked) {
+		enq_flags |= SCX_ENQ_BLOCKED;
+	} else {
+		/* see %SCX_OPS_ENQ_EXITING */
+		if (!(sch->ops.flags & SCX_OPS_ENQ_EXITING) &&
+		    unlikely(p->flags & PF_EXITING)) {
+			__scx_add_event(sch, SCX_EV_ENQ_SKIP_EXITING, 1);
+			goto local;
+		}
 
-	/* see %SCX_OPS_ENQ_MIGRATION_DISABLED */
-	if (!(sch->ops.flags & SCX_OPS_ENQ_MIGRATION_DISABLED) &&
-	    is_migration_disabled(p)) {
-		__scx_add_event(sch, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED, 1);
-		goto local;
+		/* see %SCX_OPS_ENQ_MIGRATION_DISABLED */
+		if (!(sch->ops.flags & SCX_OPS_ENQ_MIGRATION_DISABLED) &&
+		    is_migration_disabled(p)) {
+			__scx_add_event(sch, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED, 1);
+			goto local;
+		}
 	}
 
 	if (unlikely(!SCX_HAS_OP(sch, enqueue)))
@@ -2064,8 +2107,17 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_
 	int sticky_cpu = p->scx.sticky_cpu;
 	u64 enq_flags = core_enq_flags | rq->scx.remote_activate_enq_flags;
 
-	if (enq_flags & ENQUEUE_WAKEUP)
+	/*
+	 * p->is_blocked is cleared after wakeup_preempt(), so remember whether
+	 * this is a full wakeup activation. If wakeup_preempt_scx() isn't called,
+	 * set_next_task_scx() or a subsequent non-wakeup enqueue clears the flag.
+	 */
+	if (enq_flags & ENQUEUE_WAKEUP) {
 		rq->scx.flags |= SCX_RQ_IN_WAKEUP;
+		p->scx.flags |= SCX_TASK_ENQ_WAKEUP;
+	} else {
+		p->scx.flags &= ~SCX_TASK_ENQ_WAKEUP;
+	}
 
 	/*
 	 * Restoring the current scheduling context will be immediately followed
@@ -2270,11 +2322,24 @@ static void wakeup_preempt_scx(struct rq *rq, struct task_struct *p, int wake_fl
 {
 	/*
 	 * Preemption between SCX tasks is implemented by resetting the victim
-	 * task's slice to 0 and triggering reschedule on the target CPU.
-	 * Nothing to do.
+	 * task's slice to 0 and triggering reschedule on the target CPU. A
+	 * mutex-blocked task is kept queued for proxy execution, so its wakeup
+	 * doesn't go through enqueue_task_scx(). If the BPF scheduler manages
+	 * blocked donors, reschedule explicitly so that it can reconsider a
+	 * donor it declined to dispatch while blocked.
 	 */
-	if (p->sched_class == &ext_sched_class)
+	if (p->sched_class == &ext_sched_class) {
+		bool enq_wakeup = p->scx.flags & SCX_TASK_ENQ_WAKEUP;
+
+		p->scx.flags &= ~SCX_TASK_ENQ_WAKEUP;
+		if (!enq_wakeup && p->is_blocked) {
+			struct scx_sched *sch = scx_task_sched(p);
+
+			if (sch && (sch->ops.flags & SCX_OPS_ENQ_BLOCKED))
+				resched_curr(rq);
+		}
 		return;
+	}
 
 	/*
 	 * Getting preempted by a higher-priority class. Reenqueue IMMED tasks.
@@ -2389,6 +2454,20 @@ static bool task_can_run_on_remote_rq(struct scx_sched *sch,
 
 	WARN_ON_ONCE(task_cpu(p) == cpu);
 
+	/*
+	 * A blocked donor may be moved normally to select a new callback rq.
+	 * set_task_cpu() updates wake_cpu and makes the destination rq its new
+	 * callback home.
+	 *
+	 * proxy_set_task_cpu() instead preserves wake_cpu when moving a donor to
+	 * its lock owner's CPU. Keep such a donor on the proxy rq until it wakes;
+	 * otherwise normal BPF placement may repeatedly pull it back to its
+	 * callback rq only for proxy execution to move it to the owner again.
+	 */
+	if (sched_proxy_exec() && p->is_blocked &&
+	    task_cpu(p) != p->wake_cpu)
+		return false;
+
 	/*
 	 * If @p has migration disabled, @p->cpus_ptr is updated to contain only
 	 * the pinned CPU in migrate_disable_switch() while @p is being switched
@@ -3018,6 +3097,8 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 {
 	bool can_stop_tick;
 
+	p->scx.flags &= ~SCX_TASK_ENQ_WAKEUP;
+
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		/*
 		 * Core-sched might decide to execute @p before it is
@@ -3169,6 +3250,37 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		set_task_runnable(rq, p);
 
+		/*
+		 * The rq lock has remained held since scx_allow_proxy_exec(), so
+		 * @p's scheduler association cannot have changed. An associated
+		 * donor stays queued only when its BPF scheduler enables
+		 * %SCX_OPS_ENQ_BLOCKED; delegate its admission to that scheduler.
+		 *
+		 * If @sch is NULL, @p is transitioning into the root scheduler. The
+		 * root is published before tasks enter EXT and cannot be cleared while
+		 * this rq is locked. Preserve generic proxy execution by placing the
+		 * donor directly on the local DSQ.
+		 */
+		if (p->is_blocked) {
+			/*
+			 * If the donor is the same and only the mutex owner
+			 * changes, avoid triggering another ops.enqueue(): the
+			 * BPF scheduler has already admitted the donor, so it
+			 * can continue running.
+			 */
+			if (next == p)
+				goto switch_class;
+
+			if (sch) {
+				WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_BLOCKED));
+				scx_do_enqueue_task(rq, p, 0, -1);
+			} else {
+				scx_dispatch_enqueue(scx_root, rq, &rq->scx.local_dsq,
+						     p, 0);
+			}
+			goto switch_class;
+		}
+
 		/*
 		 * If @p has slice left and is being put, @p is getting
 		 * preempted by a higher priority scheduler class or core-sched
@@ -7337,6 +7449,11 @@ int scx_validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
 		return -EINVAL;
 	}
 
+	if ((ops->flags & SCX_OPS_ENQ_BLOCKED) && !ops->enqueue) {
+		scx_error(sch, "SCX_OPS_ENQ_BLOCKED requires ops.enqueue() to be implemented");
+		return -EINVAL;
+	}
+
 	/*
 	 * SCX_OPS_TID_TO_TASK is enabled by the root scheduler. A sub-sched
 	 * may set it to declare a dependency; reject if the root hasn't
@@ -7723,6 +7840,10 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 
 		if (old_class != new_class)
 			queue_flags |= DEQUEUE_CLASS;
+		if (old_class == new_class && new_class == &ext_sched_class) {
+			scx_prepare_task_sched_change(p, sch);
+			queue_flags |= DEQUEUE_NOCLOCK;
+		}
 
 		scoped_guard (sched_change, p, new_class, queue_flags) {
 			set_task_slice(p, READ_ONCE(sch->slice_dfl));
diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index 351ffd1e02fb8..800a29ca6069a 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -213,6 +213,19 @@ enum scx_ops_flags {
 	 */
 	SCX_OPS_TID_TO_TASK		= 1LLU << 8,
 
+	/*
+	 * If set, mutex-blocked tasks remain runnable as proxy donors and are
+	 * passed to ops.enqueue() with %SCX_ENQ_BLOCKED. The BPF scheduler controls
+	 * when donors are dispatched and whether they should preempt other work.
+	 *
+	 * If clear, mutex-blocked tasks are removed from the runqueue normally
+	 * and cannot donate their scheduling context through proxy execution.
+	 *
+	 * For blocked donors, this flag takes precedence over
+	 * %SCX_OPS_ENQ_EXITING and %SCX_OPS_ENQ_MIGRATION_DISABLED.
+	 */
+	SCX_OPS_ENQ_BLOCKED		= 1LLU << 9,
+
 	SCX_OPS_ALL_FLAGS		= SCX_OPS_KEEP_BUILTIN_IDLE |
 					  SCX_OPS_ENQ_LAST |
 					  SCX_OPS_ENQ_EXITING |
@@ -221,7 +234,8 @@ enum scx_ops_flags {
 					  SCX_OPS_SWITCH_PARTIAL |
 					  SCX_OPS_BUILTIN_IDLE_PER_NODE |
 					  SCX_OPS_ALWAYS_ENQ_IMMED |
-					  SCX_OPS_TID_TO_TASK,
+					  SCX_OPS_TID_TO_TASK |
+					  SCX_OPS_ENQ_BLOCKED,
 
 	/* high 8 bits are internal, don't include in SCX_OPS_ALL_FLAGS */
 	__SCX_OPS_INTERNAL_MASK		= 0xffLLU << 56,
@@ -1651,6 +1665,12 @@ enum scx_enq_flags {
 	 */
 	SCX_ENQ_LAST		= 1LLU << 41,
 
+	/*
+	 * The task is blocked on a mutex and is being kept runnable as a proxy
+	 * donor. Only passed to ops.enqueue() when %SCX_OPS_ENQ_BLOCKED is set.
+	 */
+	SCX_ENQ_BLOCKED		= 1LLU << 42,
+
 	/* high 8 bits are internal */
 	__SCX_ENQ_INTERNAL_MASK	= 0xffLLU << 56,
 
@@ -1947,6 +1967,7 @@ void scx_task_iter_start(struct scx_task_iter *iter, struct cgroup *cgrp);
 void scx_task_iter_unlock(struct scx_task_iter *iter);
 void scx_task_iter_stop(struct scx_task_iter *iter);
 struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter);
+void scx_prepare_task_sched_change(struct task_struct *p, struct scx_sched *sch);
 void scx_dispatch_dequeue(struct rq *rq, struct task_struct *p);
 void scx_do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
 			 int sticky_cpu);
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index f4ce5b74b7bfc..b4055a1044537 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -748,8 +748,9 @@ static void scx_rehome_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_held(&p->pi_lock);
 	lockdep_assert_rq_held(task_rq(p));
 
+	scx_prepare_task_sched_change(p, to);
 	scoped_guard (sched_change, p, p->sched_class,
-		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
+		      DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_state(p, SCX_TASK_INIT_BEGIN);
 		scx_set_task_state(p, SCX_TASK_INIT);
@@ -779,8 +780,9 @@ static void scx_punt_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_rq_held(task_rq(p));
 	WARN_ON_ONCE(!READ_ONCE(to->bypass_depth));
 
+	scx_prepare_task_sched_change(p, to);
 	scoped_guard (sched_change, p, p->sched_class,
-		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
+		      DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_sched(p, to);
 	}
diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
index 7757252d52e21..0e83ebfbee2b5 100644
--- a/tools/sched_ext/include/scx/compat.h
+++ b/tools/sched_ext/include/scx/compat.h
@@ -117,6 +117,7 @@ static inline bool __COMPAT_struct_has_field(const char *type, const char *field
 #define SCX_OPS_ALLOW_QUEUED_WAKEUP SCX_OPS_FLAG(SCX_OPS_ALLOW_QUEUED_WAKEUP)
 #define SCX_OPS_BUILTIN_IDLE_PER_NODE SCX_OPS_FLAG(SCX_OPS_BUILTIN_IDLE_PER_NODE)
 #define SCX_OPS_ALWAYS_ENQ_IMMED SCX_OPS_FLAG(SCX_OPS_ALWAYS_ENQ_IMMED)
+#define SCX_OPS_ENQ_BLOCKED SCX_OPS_FLAG(SCX_OPS_ENQ_BLOCKED)
 
 #define SCX_PICK_IDLE_FLAG(name) __COMPAT_ENUM_OR_ZERO("scx_pick_idle_cpu_flags", #name)
 
diff --git a/tools/sched_ext/include/scx/enum_defs.autogen.h b/tools/sched_ext/include/scx/enum_defs.autogen.h
index da4b459820fdd..79b31eb7db7cb 100644
--- a/tools/sched_ext/include/scx/enum_defs.autogen.h
+++ b/tools/sched_ext/include/scx/enum_defs.autogen.h
@@ -55,6 +55,7 @@
 #define HAVE_SCX_ENQ_IMMED
 #define HAVE_SCX_ENQ_REENQ
 #define HAVE_SCX_ENQ_LAST
+#define HAVE_SCX_ENQ_BLOCKED
 #define HAVE___SCX_ENQ_INTERNAL_MASK
 #define HAVE_SCX_ENQ_CLEAR_OPSS
 #define HAVE_SCX_ENQ_DSQ_PRIQ
diff --git a/tools/sched_ext/include/scx/enums.autogen.bpf.h b/tools/sched_ext/include/scx/enums.autogen.bpf.h
index dafccbb6b69d2..7efe7b9346b49 100644
--- a/tools/sched_ext/include/scx/enums.autogen.bpf.h
+++ b/tools/sched_ext/include/scx/enums.autogen.bpf.h
@@ -130,6 +130,9 @@ const volatile u64 __SCX_ENQ_REENQ __weak;
 const volatile u64 __SCX_ENQ_LAST __weak;
 #define SCX_ENQ_LAST __SCX_ENQ_LAST
 
+const volatile u64 __SCX_ENQ_BLOCKED __weak;
+#define SCX_ENQ_BLOCKED __SCX_ENQ_BLOCKED
+
 const volatile u64 __SCX_ENQ_CLEAR_OPSS __weak;
 #define SCX_ENQ_CLEAR_OPSS __SCX_ENQ_CLEAR_OPSS
 
diff --git a/tools/sched_ext/include/scx/enums.autogen.h b/tools/sched_ext/include/scx/enums.autogen.h
index bbd4901f4fce3..f8fbeb7fbf95b 100644
--- a/tools/sched_ext/include/scx/enums.autogen.h
+++ b/tools/sched_ext/include/scx/enums.autogen.h
@@ -47,6 +47,7 @@
 	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_IMMED); \
 	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_REENQ); \
 	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_LAST); \
+	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_BLOCKED); \
 	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_CLEAR_OPSS); \
 	SCX_ENUM_SET(skel, scx_enq_flags, SCX_ENQ_DSQ_PRIQ); \
 	SCX_ENUM_SET(skel, scx_deq_flags, SCX_DEQ_SCHED_CHANGE); \
-- 
2.55.0

[PATCH 12/14] sched_ext: Add selftest for blocked donor admission

[Andrea Righi]

SCX_OPS_ENQ_BLOCKED allows BPF schedulers to receive blocked proxy
donors through ops.enqueue(). SCX_ENQ_BLOCKED identifies blocked-donor
admission requests. Add selftest coverage for this interface.

Exercise a priority inversion using a weighted-vruntime BPF scheduler.
A nice +19 owner holds a shared mutex, a nice -20 donor blocks on it,
and nice 0 CPU contenders, one per allowed CPU, keep the system busy.
Test both a same-CPU topology and a cross-CPU topology with the donor
and owner on different CPUs.

Treat blocked donors according to the normal BPF ordering policy and
assign the default slice on every enqueue, as for other tasks. Run each
CPU placement configuration with SCX_OPS_ENQ_BLOCKED first disabled and
then enabled, count blocked-donor enqueues by CPU and report average
mutex hold and wait times. Verify that full wakeups are never reported
as blocked-donor admissions by checking that SCX_ENQ_WAKEUP and
SCX_ENQ_BLOCKED are not set together.

Proxy execution coverage requires CONFIG_SCHED_PROXY_EXEC=y, which the
selftest config selects. Access to the kernel mutex is provided via a
loadable kernel module, built through TEST_GEN_MODS_DIR and managed by
the test.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 tools/testing/selftests/sched_ext/.gitignore  |   4 +
 tools/testing/selftests/sched_ext/Makefile    |   2 +
 tools/testing/selftests/sched_ext/config      |   2 +
 .../selftests/sched_ext/enq_blocked.bpf.c     | 116 +++
 .../testing/selftests/sched_ext/enq_blocked.c | 918 ++++++++++++++++++
 .../testing/selftests/sched_ext/enq_blocked.h |  28 +
 .../selftests/sched_ext/test_modules/Makefile |  13 +
 .../test_modules/scx_enq_blocked_test.c       | 195 ++++
 8 files changed, 1278 insertions(+)
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.c
 create mode 100644 tools/testing/selftests/sched_ext/enq_blocked.h
 create mode 100644 tools/testing/selftests/sched_ext/test_modules/Makefile
 create mode 100644 tools/testing/selftests/sched_ext/test_modules/scx_enq_blocked_test.c

diff --git a/tools/testing/selftests/sched_ext/.gitignore b/tools/testing/selftests/sched_ext/.gitignore
index ae5491a114c09..54a1fd2af713d 100644
--- a/tools/testing/selftests/sched_ext/.gitignore
+++ b/tools/testing/selftests/sched_ext/.gitignore
@@ -4,3 +4,7 @@
 !Makefile
 !.gitignore
 !config
+!test_modules/
+!test_modules/scx_enq_blocked_test.c
+!test_modules/Makefile
+test_modules/*.mod.c
diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index 3cfe90e0f34fa..51a16b3d32d9b 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -5,6 +5,7 @@ include ../../../scripts/Makefile.arch
 include ../../../scripts/Makefile.include
 
 TEST_GEN_PROGS := runner
+TEST_GEN_MODS_DIR := test_modules
 
 # override lib.mk's default rules
 OVERRIDE_TARGETS := 1
@@ -164,6 +165,7 @@ all_test_bpfprogs := $(foreach prog,$(wildcard *.bpf.c),$(INCLUDE_DIR)/$(patsubs
 auto-test-targets :=			\
 	create_dsq			\
 	dequeue				\
+	enq_blocked			\
 	enq_last_no_enq_fails		\
 	ddsp_bogus_dsq_fail		\
 	ddsp_vtimelocal_fail		\
diff --git a/tools/testing/selftests/sched_ext/config b/tools/testing/selftests/sched_ext/config
index aa901b05c8ad6..affa3cf33470a 100644
--- a/tools/testing/selftests/sched_ext/config
+++ b/tools/testing/selftests/sched_ext/config
@@ -6,3 +6,5 @@ CONFIG_BPF=y
 CONFIG_BPF_SYSCALL=y
 CONFIG_DEBUG_INFO=y
 CONFIG_DEBUG_INFO_BTF=y
+CONFIG_EXPERT=y
+CONFIG_SCHED_PROXY_EXEC=y
diff --git a/tools/testing/selftests/sched_ext/enq_blocked.bpf.c b/tools/testing/selftests/sched_ext/enq_blocked.bpf.c
new file mode 100644
index 0000000000000..212690bf4e07b
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/enq_blocked.bpf.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+ *
+ * Verify that SCX_OPS_ENQ_BLOCKED passes blocked proxy donors through
+ * ops.enqueue() and record whether callbacks occur on the donor or owner CPU.
+ */
+
+#include <scx/common.bpf.h>
+
+#define SHARED_DSQ 0
+
+char _license[] SEC("license") = "GPL";
+
+s32 donor_pid;
+s32 donor_cpu = -1;
+s32 owner_cpu = -1;
+u64 nr_blocked_enqueues;
+u64 nr_blocked_enqueues_donor_cpu;
+u64 nr_blocked_enqueues_owner_cpu;
+u64 nr_blocked_enqueues_other_cpu;
+u64 nr_blocked_wakeups;
+static u64 vtime_now;
+
+UEI_DEFINE(uei);
+
+s32 BPF_STRUCT_OPS(enq_blocked_select_cpu,
+		   struct task_struct *p, s32 prev_cpu, u64 wake_flags)
+{
+	return prev_cpu;
+}
+
+void BPF_STRUCT_OPS(enq_blocked_enqueue, struct task_struct *p, u64 enq_flags)
+{
+	u64 vtime = p->scx.dsq_vtime;
+
+	if (enq_flags & SCX_ENQ_BLOCKED) {
+		int cpu = scx_bpf_task_cpu(p);
+
+		if (enq_flags & SCX_ENQ_WAKEUP)
+			__sync_fetch_and_add(&nr_blocked_wakeups, 1);
+
+		if (p->pid == donor_pid) {
+			__sync_fetch_and_add(&nr_blocked_enqueues, 1);
+			if (cpu == donor_cpu)
+				__sync_fetch_and_add(&nr_blocked_enqueues_donor_cpu, 1);
+			else if (cpu == owner_cpu)
+				__sync_fetch_and_add(&nr_blocked_enqueues_owner_cpu, 1);
+			else
+				__sync_fetch_and_add(&nr_blocked_enqueues_other_cpu, 1);
+		}
+	}
+
+	/* Limit the amount of budget an idling task can accumulate. */
+	if (time_before(vtime, vtime_now - SCX_SLICE_DFL))
+		vtime = vtime_now - SCX_SLICE_DFL;
+
+	scx_bpf_dsq_insert_vtime(p, SHARED_DSQ, SCX_SLICE_DFL, vtime,
+				 enq_flags);
+	scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_IDLE);
+}
+
+void BPF_STRUCT_OPS(enq_blocked_dispatch, s32 cpu, struct task_struct *prev)
+{
+	scx_bpf_dsq_move_to_local(SHARED_DSQ, 0);
+}
+
+void BPF_STRUCT_OPS(enq_blocked_running, struct task_struct *p)
+{
+	if (time_before(vtime_now, p->scx.dsq_vtime))
+		vtime_now = p->scx.dsq_vtime;
+}
+
+void BPF_STRUCT_OPS(enq_blocked_stopping, struct task_struct *p, bool runnable)
+{
+	u64 delta = scale_by_task_weight_inverse(p,
+					 SCX_SLICE_DFL - p->scx.slice);
+
+	scx_bpf_task_set_dsq_vtime(p, p->scx.dsq_vtime + delta);
+}
+
+void BPF_STRUCT_OPS(enq_blocked_enable, struct task_struct *p)
+{
+	scx_bpf_task_set_dsq_vtime(p, vtime_now);
+}
+
+s32 BPF_STRUCT_OPS_SLEEPABLE(enq_blocked_init)
+{
+	int ret;
+
+	ret = scx_bpf_create_dsq(SHARED_DSQ, -1);
+	if (ret) {
+		scx_bpf_error("failed to create DSQ %d (%d)", SHARED_DSQ, ret);
+		return ret;
+	}
+
+	return 0;
+}
+
+void BPF_STRUCT_OPS(enq_blocked_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops enq_blocked_ops = {
+	.select_cpu		= (void *)enq_blocked_select_cpu,
+	.enqueue		= (void *)enq_blocked_enqueue,
+	.dispatch		= (void *)enq_blocked_dispatch,
+	.running		= (void *)enq_blocked_running,
+	.stopping		= (void *)enq_blocked_stopping,
+	.enable			= (void *)enq_blocked_enable,
+	.init			= (void *)enq_blocked_init,
+	.exit			= (void *)enq_blocked_exit,
+	.name			= "enq_blocked",
+};
diff --git a/tools/testing/selftests/sched_ext/enq_blocked.c b/tools/testing/selftests/sched_ext/enq_blocked.c
new file mode 100644
index 0000000000000..896c66820a744
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/enq_blocked.c
@@ -0,0 +1,918 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+ *
+ * Exercise a priority inversion with the owner and donor first pinned to the
+ * same CPU, then with each on a different CPU. A high-priority donor blocks on
+ * a mutex held by a low-priority owner while one medium-priority contender per
+ * available CPU keeps the system busy. A weighted-vruntime BPF scheduler runs
+ * both CPU placement configurations with SCX_OPS_ENQ_BLOCKED first disabled
+ * and then enabled. The test validates blocked-donor admission and reports the
+ * average mutex hold and wait times, plus their enabled-minus-disabled deltas,
+ * for each configuration. The timing data is informational.
+ *
+ * CONFIG_SCHED_PROXY_EXEC=y is required to exercise the proxy-execution paths.
+ */
+#define _GNU_SOURCE
+
+#include <bpf/bpf.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <pthread.h>
+#include <sched.h>
+#include <scx/common.h>
+#include <stdatomic.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "enq_blocked.bpf.skel.h"
+#include "enq_blocked.h"
+#include "scx_test.h"
+
+#define MODULE_NAME	"scx_enq_blocked_test"
+#define MODULE_FILE	"test_modules/" MODULE_NAME ".ko"
+#define DEVICE_PATH	"/dev/scx_enq_blocked"
+#define WAIT_STEP_US	1000
+#define WAIT_TIMEOUT_MS	2000
+#define NR_WARMUP_TRIALS	1
+#define NR_MEASURED_TRIALS	10
+#define NR_TRIALS	(NR_WARMUP_TRIALS + NR_MEASURED_TRIALS)
+#define JOIN_TIMEOUT_MS	((NR_TRIALS + 1) * WAIT_TIMEOUT_MS)
+#define OWNER_NICE	19
+#define DONOR_NICE	-20
+#define CONTENDER_NICE	0
+
+struct thread_ctx {
+	atomic_bool start_donor;
+	atomic_bool abort;
+	atomic_bool stop_contender;
+	atomic_bool measurement_ready;
+	atomic_int donor_pid;
+	atomic_int donor_completed;
+	int fd;
+	int donor_cpu;
+	int owner_cpu;
+};
+
+struct contender_ctx {
+	struct thread_ctx *thread_ctx;
+	atomic_int status;
+	int cpu;
+};
+
+struct run_result {
+	struct enq_blocked_stats stats;
+	u64 nr_blocked_enqueues;
+	u64 nr_blocked_enqueues_donor_cpu;
+	u64 nr_blocked_enqueues_owner_cpu;
+	u64 nr_blocked_enqueues_other_cpu;
+	u64 nr_blocked_wakeups;
+};
+
+static bool parse_bool(const char *value, bool *result)
+{
+	if (!strcasecmp(value, "1") || !strcasecmp(value, "y") ||
+	    !strcasecmp(value, "yes") || !strcasecmp(value, "on") ||
+	    !strcasecmp(value, "true")) {
+		*result = true;
+		return true;
+	}
+
+	if (!strcasecmp(value, "0") || !strcasecmp(value, "n") ||
+	    !strcasecmp(value, "no") || !strcasecmp(value, "off") ||
+	    !strcasecmp(value, "false")) {
+		*result = false;
+		return true;
+	}
+
+	return false;
+}
+
+static bool cmdline_bool(const char *name, bool default_value)
+{
+	char cmdline[4096], *newline, *saveptr = NULL, *token;
+	size_t name_len = strlen(name);
+	bool value = default_value;
+	FILE *file;
+
+	file = fopen("/proc/cmdline", "r");
+	if (!file)
+		return default_value;
+
+	if (!fgets(cmdline, sizeof(cmdline), file)) {
+		fclose(file);
+		return default_value;
+	}
+	fclose(file);
+	newline = strchr(cmdline, '\n');
+	if (newline)
+		*newline = '\0';
+
+	for (token = strtok_r(cmdline, " ", &saveptr); token;
+	     token = strtok_r(NULL, " ", &saveptr)) {
+		bool parsed;
+
+		if (strncmp(token, name, name_len) || token[name_len] != '=')
+			continue;
+		if (parse_bool(token + name_len + 1, &parsed))
+			value = parsed;
+	}
+
+	return value;
+}
+
+static int module_path(char *path, size_t size)
+{
+	ssize_t len;
+	char *slash;
+
+	len = readlink("/proc/self/exe", path, size - 1);
+	if (len < 0)
+		return -errno;
+	path[len] = '\0';
+
+	slash = strrchr(path, '/');
+	if (!slash)
+		return -EINVAL;
+	*slash = '\0';
+
+	if (snprintf(slash, size - (slash - path), "/%s", MODULE_FILE) >=
+	    size - (slash - path))
+		return -ENAMETOOLONG;
+
+	return 0;
+}
+
+static int load_test_module(bool *loaded_here)
+{
+	char path[PATH_MAX];
+	int fd, err;
+
+	err = module_path(path, sizeof(path));
+	if (err)
+		return err;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return -errno;
+
+	if (syscall(SYS_finit_module, fd, "", 0)) {
+		err = errno;
+		close(fd);
+		if (err == EEXIST)
+			return 0;
+		return -err;
+	}
+
+	close(fd);
+	*loaded_here = true;
+	return 0;
+}
+
+static void unload_test_module(bool loaded_here)
+{
+	if (loaded_here && syscall(SYS_delete_module, MODULE_NAME, O_NONBLOCK))
+		SCX_ERR("Failed to unload %s (%d)", MODULE_NAME, errno);
+}
+
+static int pin_to_cpu(int cpu)
+{
+	cpu_set_t mask;
+
+	CPU_ZERO(&mask);
+	CPU_SET(cpu, &mask);
+	return sched_setaffinity(0, sizeof(mask), &mask) ? errno : 0;
+}
+
+static int select_test_cpus(bool cross_cpu, cpu_set_t *mask, int *donor_cpu,
+			    int *owner_cpu)
+{
+	int cpu, first = -1;
+
+	if (sched_getaffinity(0, sizeof(*mask), mask))
+		return -errno;
+
+	for (cpu = 0; cpu < CPU_SETSIZE; cpu++) {
+		if (!CPU_ISSET(cpu, mask))
+			continue;
+		if (first < 0) {
+			first = cpu;
+			if (!cross_cpu)
+				break;
+		} else {
+			*donor_cpu = first;
+			*owner_cpu = cpu;
+			return 0;
+		}
+	}
+
+	if (first < 0)
+		return -ENODEV;
+	if (cross_cpu)
+		return -EAGAIN;
+
+	*donor_cpu = first;
+	*owner_cpu = first;
+	return 0;
+}
+
+static int set_nice(int nice)
+{
+	return setpriority(PRIO_PROCESS, 0, nice) ? errno : 0;
+}
+
+static bool wait_for_pid(atomic_int *pid)
+{
+	int waited_ms;
+
+	for (waited_ms = 0; waited_ms < WAIT_TIMEOUT_MS; waited_ms++) {
+		if (atomic_load_explicit(pid, memory_order_acquire) > 0)
+			return true;
+		usleep(WAIT_STEP_US);
+	}
+
+	return false;
+}
+
+static int wait_for_contenders(struct contender_ctx *contenders,
+			       size_t nr_contenders)
+{
+	size_t i, nr_ready;
+	int status, waited_ms;
+
+	for (waited_ms = 0; waited_ms < WAIT_TIMEOUT_MS; waited_ms++) {
+		nr_ready = 0;
+		for (i = 0; i < nr_contenders; i++) {
+			status = atomic_load_explicit(&contenders[i].status,
+						      memory_order_acquire);
+			if (status < 0)
+				return status;
+			if (status > 0)
+				nr_ready++;
+		}
+		if (nr_ready == nr_contenders)
+			return 1;
+		usleep(WAIT_STEP_US);
+	}
+
+	return -ETIMEDOUT;
+}
+
+static int wait_for_donor_state(struct thread_ctx *ctx, int expected)
+{
+	int state, waited_ms;
+
+	for (waited_ms = 0; waited_ms < WAIT_TIMEOUT_MS; waited_ms++) {
+		state = ioctl(ctx->fd, ENQ_BLOCKED_IOCTL_DONOR_STATE);
+		if (state == expected)
+			return state;
+		if (state < 0 && errno != ENOENT)
+			return -errno;
+		usleep(WAIT_STEP_US);
+	}
+
+	return -ETIMEDOUT;
+}
+
+static bool wait_for_donor(struct thread_ctx *ctx, int trial)
+{
+	int waited_ms;
+
+	for (waited_ms = 0; waited_ms < WAIT_TIMEOUT_MS; waited_ms++) {
+		if (atomic_load_explicit(&ctx->donor_completed,
+					 memory_order_acquire) >= trial)
+			return true;
+		if (atomic_load_explicit(&ctx->abort, memory_order_relaxed))
+			return false;
+		usleep(WAIT_STEP_US);
+	}
+
+	return false;
+}
+
+static bool wait_for_measurement(struct thread_ctx *ctx)
+{
+	while (!atomic_load_explicit(&ctx->measurement_ready,
+				     memory_order_acquire) &&
+	       !atomic_load_explicit(&ctx->abort, memory_order_relaxed))
+		sched_yield();
+
+	return !atomic_load_explicit(&ctx->abort, memory_order_relaxed);
+}
+
+static void *contender_fn(void *arg)
+{
+	struct contender_ctx *contender = arg;
+	struct thread_ctx *ctx = contender->thread_ctx;
+	int err;
+
+	err = pin_to_cpu(contender->cpu);
+	if (!err)
+		err = set_nice(CONTENDER_NICE);
+	atomic_store_explicit(&contender->status, err ? -err : 1,
+			      memory_order_release);
+	if (err)
+		return (void *)(uintptr_t)err;
+
+	while (!atomic_load_explicit(&ctx->stop_contender,
+				     memory_order_relaxed))
+		;
+
+	return NULL;
+}
+
+static void *owner_fn(void *arg)
+{
+	struct thread_ctx *ctx = arg;
+	int err, i;
+
+	err = pin_to_cpu(ctx->owner_cpu);
+	if (err)
+		return (void *)(uintptr_t)err;
+	err = set_nice(OWNER_NICE);
+	if (err)
+		return (void *)(uintptr_t)err;
+
+	for (i = 0; i < NR_TRIALS; i++) {
+		if (ioctl(ctx->fd, ENQ_BLOCKED_IOCTL_OWNER))
+			return (void *)(uintptr_t)errno;
+		if (!wait_for_donor(ctx, i + 1))
+			return (void *)(uintptr_t)ETIMEDOUT;
+
+		if (i + 1 == NR_WARMUP_TRIALS && !wait_for_measurement(ctx))
+			return NULL;
+	}
+
+	return NULL;
+}
+
+static int run_donor_trial(struct thread_ctx *ctx)
+{
+	int waited_ms;
+
+	for (waited_ms = 0; waited_ms < WAIT_TIMEOUT_MS; waited_ms++) {
+		if (!ioctl(ctx->fd, ENQ_BLOCKED_IOCTL_DONOR))
+			return 0;
+		if (errno != EAGAIN)
+			return -errno;
+		usleep(WAIT_STEP_US);
+	}
+
+	return -ETIMEDOUT;
+}
+
+static void *donor_fn(void *arg)
+{
+	struct thread_ctx *ctx = arg;
+	int err, i;
+
+	err = pin_to_cpu(ctx->donor_cpu);
+	if (err)
+		return (void *)(uintptr_t)err;
+	err = set_nice(DONOR_NICE);
+	if (err)
+		return (void *)(uintptr_t)err;
+
+	atomic_store_explicit(&ctx->donor_pid, syscall(SYS_gettid),
+			      memory_order_release);
+	while (!atomic_load_explicit(&ctx->start_donor, memory_order_acquire) &&
+	       !atomic_load_explicit(&ctx->abort, memory_order_relaxed))
+		sched_yield();
+
+	if (atomic_load_explicit(&ctx->abort, memory_order_relaxed))
+		return NULL;
+
+	for (i = 0; i < NR_TRIALS; i++) {
+		err = run_donor_trial(ctx);
+		if (err)
+			return (void *)(uintptr_t)-err;
+		atomic_store_explicit(&ctx->donor_completed, i + 1,
+				      memory_order_release);
+	}
+
+	return NULL;
+}
+
+static void print_avg_time(const char *name, u64 total_ns, u64 samples)
+{
+	u64 avg_ns = samples ? total_ns / samples : 0;
+
+	printf("  %s_avg_ns=%llu (%llu.%03llu ms, samples=%llu)\n", name,
+	       (unsigned long long)avg_ns,
+	       (unsigned long long)(avg_ns / 1000000),
+	       (unsigned long long)((avg_ns / 1000) % 1000),
+	       (unsigned long long)samples);
+}
+
+static void print_avg_delta(const char *name, u64 disabled_total,
+			    u64 disabled_samples, u64 enabled_total,
+			    u64 enabled_samples)
+{
+	u64 disabled_avg, enabled_avg;
+	s64 delta_ns;
+	double delta_pct;
+
+	if (!disabled_samples || !enabled_samples)
+		return;
+
+	disabled_avg = disabled_total / disabled_samples;
+	enabled_avg = enabled_total / enabled_samples;
+	delta_ns = (s64)enabled_avg - (s64)disabled_avg;
+	delta_pct = disabled_avg ? 100.0 * delta_ns / disabled_avg : 0.0;
+
+	printf("  %s_delta_ns=%+lld (%+.2f%%)\n", name,
+	       (long long)delta_ns, delta_pct);
+}
+
+static int join_thread(pthread_t thread, const struct timespec *deadline,
+		       int *thread_err)
+{
+	void *result;
+	int err;
+
+	err = pthread_timedjoin_np(thread, &result, deadline);
+	if (err)
+		return err;
+
+	*thread_err = (int)(uintptr_t)result;
+	return 0;
+}
+
+static void set_join_deadline(struct timespec *deadline)
+{
+	clock_gettime(CLOCK_REALTIME, deadline);
+	deadline->tv_sec += JOIN_TIMEOUT_MS / 1000;
+	deadline->tv_nsec += (JOIN_TIMEOUT_MS % 1000) * 1000000;
+	if (deadline->tv_nsec >= 1000000000) {
+		deadline->tv_sec++;
+		deadline->tv_nsec -= 1000000000;
+	}
+}
+
+static enum scx_test_status setup(void **ctx)
+{
+	struct enq_blocked *skel;
+	u64 flag;
+
+	skel = enq_blocked__open();
+	SCX_FAIL_IF(!skel, "Failed to open skel");
+	SCX_ENUM_INIT(skel);
+
+	flag = SCX_OPS_ENQ_BLOCKED;
+	if (!flag) {
+		enq_blocked__destroy(skel);
+		fprintf(stderr, "SKIP: SCX_OPS_ENQ_BLOCKED is unavailable\n");
+		return SCX_TEST_SKIP;
+	}
+
+	enq_blocked__destroy(skel);
+	*ctx = NULL;
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run_one(bool enq_blocked, bool cross_cpu,
+				    struct run_result *result)
+{
+	struct enq_blocked *skel;
+	struct thread_ctx thread_ctx = {};
+	struct contender_ctx *contender_ctxs = NULL;
+	struct bpf_link *link = NULL;
+	pthread_t owner, donor, *contenders = NULL;
+	struct timespec join_deadline;
+	cpu_set_t allowed_cpus;
+	bool module_loaded = false;
+	bool owner_started = false, donor_started = false;
+	bool join_timed_out = false;
+	bool proxy_enabled;
+	enum scx_test_status status = SCX_TEST_PASS;
+	int cpu, donor_pid, donor_state, err, thread_err;
+	size_t i, nr_contenders, nr_contenders_started = 0;
+	size_t nr_contenders_joined = 0;
+	u64 nr_blocked, nr_blocked_donor_cpu, nr_blocked_owner_cpu;
+	u64 nr_blocked_other_cpu, nr_blocked_wakeups;
+	struct enq_blocked_stats stats;
+
+	err = select_test_cpus(cross_cpu, &allowed_cpus, &thread_ctx.donor_cpu,
+			       &thread_ctx.owner_cpu);
+	if (err == -EAGAIN) {
+		fprintf(stderr, "SKIP: cross-CPU case requires two allowed CPUs\n");
+		return SCX_TEST_SKIP;
+	}
+	if (err) {
+		SCX_ERR("Failed to select test CPUs (%d)", -err);
+		return SCX_TEST_FAIL;
+	}
+	nr_contenders = CPU_COUNT(&allowed_cpus);
+	contenders = calloc(nr_contenders, sizeof(*contenders));
+	contender_ctxs = calloc(nr_contenders, sizeof(*contender_ctxs));
+	if (!contenders || !contender_ctxs) {
+		SCX_ERR("Failed to allocate %zu contender threads", nr_contenders);
+		status = SCX_TEST_FAIL;
+		goto out_contenders;
+	}
+
+	skel = enq_blocked__open();
+	if (!skel) {
+		SCX_ERR("Failed to open skel");
+		status = SCX_TEST_FAIL;
+		goto out_contenders;
+	}
+	SCX_ENUM_INIT(skel);
+	skel->struct_ops.enq_blocked_ops->flags =
+		SCX_OPS_ENQ_LAST |
+		(enq_blocked ? SCX_OPS_ENQ_BLOCKED : 0);
+	if (enq_blocked__load(skel)) {
+		SCX_ERR("Failed to load skel");
+		status = SCX_TEST_FAIL;
+		goto out_skel;
+	}
+
+	err = load_test_module(&module_loaded);
+	if (err == -EPERM || err == -ENOENT) {
+		fprintf(stderr, "SKIP: cannot load mutex fixture (%d)\n", -err);
+		status = SCX_TEST_SKIP;
+		goto out_skel;
+	}
+	if (err) {
+		SCX_ERR("Failed to load mutex fixture (%d)", -err);
+		status = SCX_TEST_FAIL;
+		goto out_skel;
+	}
+
+	thread_ctx.fd = open(DEVICE_PATH, O_RDONLY | O_CLOEXEC);
+	if (thread_ctx.fd < 0) {
+		SCX_ERR("Failed to open %s (%d)", DEVICE_PATH, errno);
+		status = SCX_TEST_FAIL;
+		goto out_module;
+	}
+	err = ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_PROXY_SUPPORTED);
+	if (err < 0) {
+		SCX_ERR("Failed to query proxy-exec support (%d)", errno);
+		status = SCX_TEST_FAIL;
+		goto out_fd;
+	}
+	proxy_enabled = err && cmdline_bool("sched_proxy_exec", true);
+	if (!proxy_enabled) {
+		fprintf(stderr, "SKIP: proxy execution is not enabled\n");
+		status = SCX_TEST_SKIP;
+		goto out_fd;
+	}
+	if (ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_RESET_STATS)) {
+		SCX_ERR("Failed to reset mutex statistics (%d)", errno);
+		status = SCX_TEST_FAIL;
+		goto out_fd;
+	}
+
+	if (ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_PREP_ATTACH)) {
+		SCX_ERR("Failed to prepare scheduler attachment (%d)", errno);
+		status = SCX_TEST_FAIL;
+		goto out_fd;
+	}
+
+	err = pthread_create(&owner, NULL, owner_fn, &thread_ctx);
+	if (err) {
+		SCX_ERR("Failed to create owner thread (%d)", err);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+	owner_started = true;
+
+	err = pthread_create(&donor, NULL, donor_fn, &thread_ctx);
+	if (err) {
+		SCX_ERR("Failed to create donor thread (%d)", err);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+	donor_started = true;
+
+	if (!wait_for_pid(&thread_ctx.donor_pid)) {
+		SCX_ERR("Timed out waiting for donor thread");
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	donor_pid = atomic_load_explicit(&thread_ctx.donor_pid,
+					 memory_order_acquire);
+	skel->bss->donor_pid = donor_pid;
+	skel->data->donor_cpu = thread_ctx.donor_cpu;
+	skel->data->owner_cpu = thread_ctx.owner_cpu;
+	atomic_store_explicit(&thread_ctx.start_donor, true,
+			      memory_order_release);
+
+	donor_state = ENQ_BLOCKED_DONOR_SLEEPING;
+	if (proxy_enabled)
+		donor_state |= ENQ_BLOCKED_DONOR_ON_RQ;
+	err = wait_for_donor_state(&thread_ctx, donor_state);
+	if (err < 0) {
+		SCX_ERR("Donor did not block before scheduler attachment (%d)", -err);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	link = bpf_map__attach_struct_ops(skel->maps.enq_blocked_ops);
+	if (!link) {
+		SCX_ERR("Failed to attach scheduler");
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	donor_state = ENQ_BLOCKED_DONOR_SLEEPING;
+	if (proxy_enabled && enq_blocked)
+		donor_state |= ENQ_BLOCKED_DONOR_ON_RQ;
+	err = wait_for_donor_state(&thread_ctx, donor_state);
+	if (err < 0) {
+		SCX_ERR("Unexpected donor state after scheduler attachment (%d)",
+			-err);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	if (ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_ATTACH_DONE)) {
+		SCX_ERR("Failed to complete scheduler attachment (%d)", errno);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	i = 0;
+	for (cpu = 0; cpu < CPU_SETSIZE; cpu++) {
+		if (!CPU_ISSET(cpu, &allowed_cpus))
+			continue;
+
+		contender_ctxs[i].thread_ctx = &thread_ctx;
+		contender_ctxs[i].cpu = cpu;
+		atomic_init(&contender_ctxs[i].status, 0);
+		err = pthread_create(&contenders[i], NULL, contender_fn,
+				     &contender_ctxs[i]);
+		if (err) {
+			SCX_ERR("Failed to create contender for CPU %d (%d)",
+				cpu, err);
+			status = SCX_TEST_FAIL;
+			goto out;
+		}
+		nr_contenders_started++;
+		i++;
+	}
+
+	err = wait_for_contenders(contender_ctxs, nr_contenders);
+	if (err != 1) {
+		SCX_ERR("Contender threads failed (%d)", -err);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+
+	/*
+	 * The first trial spans scheduler attachment and validates the state
+	 * transition, but including it would skew scheduling latency. Exclude it
+	 * from both the mutex and BPF enqueue measurements.
+	 */
+	if (!wait_for_donor(&thread_ctx, NR_WARMUP_TRIALS)) {
+		SCX_ERR("Timed out waiting for warm-up trial");
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+	if (ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_RESET_STATS)) {
+		SCX_ERR("Failed to reset mutex statistics after warm-up (%d)",
+			errno);
+		status = SCX_TEST_FAIL;
+		goto out;
+	}
+	skel->bss->nr_blocked_enqueues = 0;
+	skel->bss->nr_blocked_enqueues_donor_cpu = 0;
+	skel->bss->nr_blocked_enqueues_owner_cpu = 0;
+	skel->bss->nr_blocked_enqueues_other_cpu = 0;
+	skel->bss->nr_blocked_wakeups = 0;
+	atomic_store_explicit(&thread_ctx.measurement_ready, true,
+			      memory_order_release);
+
+out:
+	ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_ATTACH_DONE);
+	if (status != SCX_TEST_PASS) {
+		atomic_store_explicit(&thread_ctx.abort, true, memory_order_release);
+		atomic_store_explicit(&thread_ctx.start_donor, true,
+				      memory_order_release);
+		atomic_store_explicit(&thread_ctx.measurement_ready, true,
+				      memory_order_release);
+	}
+
+	set_join_deadline(&join_deadline);
+	if (donor_started) {
+		err = join_thread(donor, &join_deadline, &thread_err);
+		if (err == ETIMEDOUT) {
+			SCX_ERR("Timed out waiting for donor thread");
+			join_timed_out = true;
+			status = SCX_TEST_FAIL;
+		} else if (err) {
+			SCX_ERR("Failed to join donor thread (%d)", err);
+			status = SCX_TEST_FAIL;
+		} else {
+			donor_started = false;
+			if (thread_err) {
+				SCX_ERR("Donor thread failed (%d)", thread_err);
+				status = SCX_TEST_FAIL;
+			}
+		}
+	}
+	if (!join_timed_out && owner_started) {
+		err = join_thread(owner, &join_deadline, &thread_err);
+		if (err == ETIMEDOUT) {
+			SCX_ERR("Timed out waiting for owner thread");
+			join_timed_out = true;
+			status = SCX_TEST_FAIL;
+		} else if (err) {
+			SCX_ERR("Failed to join owner thread (%d)", err);
+			status = SCX_TEST_FAIL;
+		} else {
+			owner_started = false;
+			if (thread_err) {
+				SCX_ERR("Owner thread failed (%d)", thread_err);
+				status = SCX_TEST_FAIL;
+			}
+		}
+	}
+	atomic_store_explicit(&thread_ctx.stop_contender, true,
+			      memory_order_release);
+	for (i = 0; !join_timed_out && i < nr_contenders_started; i++) {
+		err = join_thread(contenders[i], &join_deadline, &thread_err);
+		if (err == ETIMEDOUT) {
+			SCX_ERR("Timed out waiting for contender on CPU %d",
+				contender_ctxs[i].cpu);
+			join_timed_out = true;
+			status = SCX_TEST_FAIL;
+		} else if (err) {
+			SCX_ERR("Failed to join contender on CPU %d (%d)",
+				contender_ctxs[i].cpu, err);
+			status = SCX_TEST_FAIL;
+		} else {
+			nr_contenders_joined++;
+			if (thread_err) {
+				SCX_ERR("Contender on CPU %d failed (%d)",
+					contender_ctxs[i].cpu, thread_err);
+				status = SCX_TEST_FAIL;
+			}
+		}
+	}
+
+	/* Restore the fair scheduler before waiting for any stranded thread. */
+	if (join_timed_out) {
+		atomic_store_explicit(&thread_ctx.abort, true,
+				      memory_order_release);
+		if (link) {
+			bpf_link__destroy(link);
+			link = NULL;
+		}
+		if (donor_started)
+			pthread_join(donor, NULL);
+		if (owner_started)
+			pthread_join(owner, NULL);
+		for (i = nr_contenders_joined;
+		     i < nr_contenders_started; i++)
+			pthread_join(contenders[i], NULL);
+	}
+
+	if (ioctl(thread_ctx.fd, ENQ_BLOCKED_IOCTL_GET_STATS, &stats)) {
+		SCX_ERR("Failed to read mutex statistics (%d)", errno);
+		status = SCX_TEST_FAIL;
+	} else {
+		result->stats = stats;
+		printf("\n[topology=%s SCX_OPS_ENQ_BLOCKED=%s]\n",
+		       cross_cpu ? "cross-cpu" : "same-cpu",
+		       enq_blocked ? "enabled" : "disabled");
+		printf("  proxy_exec=%s\n",
+		       proxy_enabled ? "enabled" : "disabled");
+		printf("  donor_cpu=%d\n", thread_ctx.donor_cpu);
+		printf("  owner_cpu=%d\n", thread_ctx.owner_cpu);
+		printf("  nr_contenders=%zu\n", nr_contenders);
+		printf("  measured_trials=%d\n", NR_MEASURED_TRIALS);
+		printf("  owner_nice=%d\n", OWNER_NICE);
+		printf("  donor_nice=%d\n", DONOR_NICE);
+		printf("  contender_nice=%d\n", CONTENDER_NICE);
+		print_avg_time("mutex_hold", stats.hold_time_ns, stats.nr_holds);
+		print_avg_time("mutex_wait", stats.wait_time_ns, stats.nr_waits);
+		if (stats.nr_holds != NR_MEASURED_TRIALS ||
+		    stats.nr_waits != NR_MEASURED_TRIALS) {
+			SCX_ERR("Expected %d measured trials, got %llu holds and %llu waits",
+				NR_MEASURED_TRIALS,
+				(unsigned long long)stats.nr_holds,
+				(unsigned long long)stats.nr_waits);
+			status = SCX_TEST_FAIL;
+		}
+	}
+
+	nr_blocked = skel->bss->nr_blocked_enqueues;
+	nr_blocked_donor_cpu = skel->bss->nr_blocked_enqueues_donor_cpu;
+	nr_blocked_owner_cpu = skel->bss->nr_blocked_enqueues_owner_cpu;
+	nr_blocked_other_cpu = skel->bss->nr_blocked_enqueues_other_cpu;
+	nr_blocked_wakeups = skel->bss->nr_blocked_wakeups;
+	result->nr_blocked_enqueues = nr_blocked;
+	result->nr_blocked_enqueues_donor_cpu = nr_blocked_donor_cpu;
+	result->nr_blocked_enqueues_owner_cpu = nr_blocked_owner_cpu;
+	result->nr_blocked_enqueues_other_cpu = nr_blocked_other_cpu;
+	result->nr_blocked_wakeups = nr_blocked_wakeups;
+	printf("  nr_blocked_enqueues=%llu\n",
+	       (unsigned long long)nr_blocked);
+	printf("  nr_blocked_enqueues_donor_cpu=%llu\n",
+	       (unsigned long long)nr_blocked_donor_cpu);
+	printf("  nr_blocked_enqueues_owner_cpu=%llu\n",
+	       (unsigned long long)nr_blocked_owner_cpu);
+	printf("  nr_blocked_enqueues_other_cpu=%llu\n",
+	       (unsigned long long)nr_blocked_other_cpu);
+	printf("  nr_blocked_wakeups=%llu\n",
+	       (unsigned long long)nr_blocked_wakeups);
+	if (status == SCX_TEST_PASS) {
+		if (enq_blocked && proxy_enabled && !nr_blocked) {
+			SCX_ERR("ops.enqueue() did not receive the blocked donor");
+			status = SCX_TEST_FAIL;
+		} else if ((!enq_blocked || !proxy_enabled) && nr_blocked) {
+			SCX_ERR("ops.enqueue() unexpectedly received %llu blocked donors",
+				(unsigned long long)nr_blocked);
+			status = SCX_TEST_FAIL;
+		} else if (nr_blocked_wakeups) {
+			SCX_ERR("Ordinary wakeups received %llu blocked enqueue flags",
+				(unsigned long long)nr_blocked_wakeups);
+			status = SCX_TEST_FAIL;
+		} else if (nr_blocked_other_cpu) {
+			SCX_ERR("Blocked donor had %llu enqueues on unexpected CPUs",
+				(unsigned long long)nr_blocked_other_cpu);
+			status = SCX_TEST_FAIL;
+		}
+	}
+
+	if (skel->data->uei.kind != EXIT_KIND(SCX_EXIT_NONE)) {
+		SCX_ERR("Scheduler exited unexpectedly (kind=%llu code=%lld)",
+			(unsigned long long)skel->data->uei.kind,
+			(long long)skel->data->uei.exit_code);
+		status = SCX_TEST_FAIL;
+	}
+
+	if (link)
+		bpf_link__destroy(link);
+out_fd:
+	close(thread_ctx.fd);
+out_module:
+	unload_test_module(module_loaded);
+out_skel:
+	enq_blocked__destroy(skel);
+out_contenders:
+	free(contender_ctxs);
+	free(contenders);
+	return status;
+}
+
+static enum scx_test_status run_topology(bool cross_cpu)
+{
+	struct run_result disabled = {}, enabled = {};
+	enum scx_test_status status;
+
+	status = run_one(false, cross_cpu, &disabled);
+	if (status != SCX_TEST_PASS)
+		return status;
+
+	status = run_one(true, cross_cpu, &enabled);
+	if (status != SCX_TEST_PASS)
+		return status;
+
+	printf("\n[topology=%s delta: enabled - disabled]\n",
+	       cross_cpu ? "cross-cpu" : "same-cpu");
+	print_avg_delta("mutex_hold", disabled.stats.hold_time_ns,
+			disabled.stats.nr_holds, enabled.stats.hold_time_ns,
+			enabled.stats.nr_holds);
+	print_avg_delta("mutex_wait", disabled.stats.wait_time_ns,
+			disabled.stats.nr_waits, enabled.stats.wait_time_ns,
+			enabled.stats.nr_waits);
+
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+	enum scx_test_status status;
+
+	(void)ctx;
+
+	status = run_topology(false);
+	if (status != SCX_TEST_PASS)
+		return status;
+
+	status = run_topology(true);
+	if (status == SCX_TEST_SKIP)
+		return SCX_TEST_PASS;
+
+	return status;
+}
+
+struct scx_test enq_blocked = {
+	.name = "enq_blocked",
+	.description = "Verify proxy donor admission under CPU-wide contention",
+	.setup = setup,
+	.run = run,
+};
+
+REGISTER_SCX_TEST(&enq_blocked)
diff --git a/tools/testing/selftests/sched_ext/enq_blocked.h b/tools/testing/selftests/sched_ext/enq_blocked.h
new file mode 100644
index 0000000000000..ef1eb97feebe6
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/enq_blocked.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES */
+#ifndef __ENQ_BLOCKED_H
+#define __ENQ_BLOCKED_H
+
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+struct enq_blocked_stats {
+	__u64 hold_time_ns;
+	__u64 wait_time_ns;
+	__u64 nr_holds;
+	__u64 nr_waits;
+};
+
+#define ENQ_BLOCKED_IOCTL_OWNER	_IO('s', 1)
+#define ENQ_BLOCKED_IOCTL_DONOR	_IO('s', 2)
+#define ENQ_BLOCKED_IOCTL_RESET_STATS	_IO('s', 3)
+#define ENQ_BLOCKED_IOCTL_GET_STATS	_IOR('s', 4, struct enq_blocked_stats)
+#define ENQ_BLOCKED_IOCTL_PREP_ATTACH	_IO('s', 5)
+#define ENQ_BLOCKED_IOCTL_ATTACH_DONE	_IO('s', 6)
+#define ENQ_BLOCKED_IOCTL_DONOR_STATE	_IO('s', 7)
+#define ENQ_BLOCKED_IOCTL_PROXY_SUPPORTED _IO('s', 8)
+
+#define ENQ_BLOCKED_DONOR_SLEEPING	(1U << 0)
+#define ENQ_BLOCKED_DONOR_ON_RQ		(1U << 1)
+
+#endif /* __ENQ_BLOCKED_H */
diff --git a/tools/testing/selftests/sched_ext/test_modules/Makefile b/tools/testing/selftests/sched_ext/test_modules/Makefile
new file mode 100644
index 0000000000000..a0e9e9401ead6
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/test_modules/Makefile
@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+
+TESTMODS_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
+KDIR ?= $(if $(O),$(O),$(realpath ../../../../..))
+
+obj-m += scx_enq_blocked_test.o
+
+all:
+	+$(Q)$(MAKE) -C $(KDIR) M=$(TESTMODS_DIR) modules
+
+clean:
+	+$(Q)$(MAKE) -C $(KDIR) M=$(TESTMODS_DIR) clean
diff --git a/tools/testing/selftests/sched_ext/test_modules/scx_enq_blocked_test.c b/tools/testing/selftests/sched_ext/test_modules/scx_enq_blocked_test.c
new file mode 100644
index 0000000000000..908689ed55786
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/test_modules/scx_enq_blocked_test.c
@@ -0,0 +1,195 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES
+ *
+ * Kernel mutex fixture for the sched_ext SCX_OPS_ENQ_BLOCKED selftest.
+ */
+
+#include <linux/atomic.h>
+#include <linux/fs.h>
+#include <linux/jiffies.h>
+#include <linux/ktime.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/sched.h>
+#include <linux/uaccess.h>
+
+#include "../enq_blocked.h"
+
+#define DONOR_WAIT_TIMEOUT	msecs_to_jiffies(2000)
+#define ATTACH_WAIT_TIMEOUT	msecs_to_jiffies(10000)
+#define MUTEX_HOLD_TIME		msecs_to_jiffies(200)
+
+static DEFINE_MUTEX(test_mutex);
+static DEFINE_SPINLOCK(donor_lock);
+static struct task_struct *donor_task;
+static atomic_t owner_ready = ATOMIC_INIT(0);
+static atomic_t donor_started = ATOMIC_INIT(0);
+static atomic_t attach_pending = ATOMIC_INIT(0);
+static atomic_t attach_done = ATOMIC_INIT(0);
+static atomic64_t hold_time_ns = ATOMIC64_INIT(0);
+static atomic64_t wait_time_ns = ATOMIC64_INIT(0);
+static atomic64_t nr_holds = ATOMIC64_INIT(0);
+static atomic64_t nr_waits = ATOMIC64_INIT(0);
+
+static long run_owner(void)
+{
+	unsigned long timeout;
+	u64 start_ns;
+	long ret = 0;
+
+	atomic_set(&donor_started, 0);
+	mutex_lock(&test_mutex);
+	start_ns = ktime_get_ns();
+	atomic_set(&owner_ready, 1);
+
+	timeout = jiffies + DONOR_WAIT_TIMEOUT;
+	while (!atomic_read(&donor_started)) {
+		if (time_after(jiffies, timeout)) {
+			ret = -ETIMEDOUT;
+			goto out;
+		}
+		cond_resched();
+	}
+	if (atomic_xchg(&attach_pending, 0)) {
+		timeout = jiffies + ATTACH_WAIT_TIMEOUT;
+		while (!atomic_read(&attach_done)) {
+			if (time_after(jiffies, timeout)) {
+				ret = -ETIMEDOUT;
+				goto out;
+			}
+			cond_resched();
+		}
+	}
+
+	/* Keep yielding while the donor blocks on test_mutex. */
+	timeout = jiffies + MUTEX_HOLD_TIME;
+	while (time_before(jiffies, timeout))
+		cond_resched();
+
+out:
+	atomic_set(&owner_ready, 0);
+	atomic64_add(ktime_get_ns() - start_ns, &hold_time_ns);
+	atomic64_inc(&nr_holds);
+	mutex_unlock(&test_mutex);
+	return ret;
+}
+
+static long run_donor(void)
+{
+	unsigned long flags;
+	u64 start_ns;
+
+	if (!atomic_read(&owner_ready))
+		return -EAGAIN;
+
+	get_task_struct(current);
+	spin_lock_irqsave(&donor_lock, flags);
+	WARN_ON_ONCE(donor_task);
+	donor_task = current;
+	spin_unlock_irqrestore(&donor_lock, flags);
+
+	atomic_set(&donor_started, 1);
+	start_ns = ktime_get_ns();
+	mutex_lock(&test_mutex);
+
+	spin_lock_irqsave(&donor_lock, flags);
+	donor_task = NULL;
+	spin_unlock_irqrestore(&donor_lock, flags);
+	put_task_struct(current);
+
+	atomic64_add(ktime_get_ns() - start_ns, &wait_time_ns);
+	atomic64_inc(&nr_waits);
+	mutex_unlock(&test_mutex);
+	return 0;
+}
+
+static long get_donor_state(void)
+{
+	struct task_struct *task;
+	unsigned long flags;
+	long state = 0;
+
+	spin_lock_irqsave(&donor_lock, flags);
+	task = donor_task;
+	if (task)
+		get_task_struct(task);
+	spin_unlock_irqrestore(&donor_lock, flags);
+	if (!task)
+		return -ENOENT;
+
+	if (READ_ONCE(task->__state) != TASK_RUNNING)
+		state |= ENQ_BLOCKED_DONOR_SLEEPING;
+	if (READ_ONCE(task->on_rq))
+		state |= ENQ_BLOCKED_DONOR_ON_RQ;
+	put_task_struct(task);
+	return state;
+}
+
+static void reset_stats(void)
+{
+	atomic64_set(&hold_time_ns, 0);
+	atomic64_set(&wait_time_ns, 0);
+	atomic64_set(&nr_holds, 0);
+	atomic64_set(&nr_waits, 0);
+}
+
+static long get_stats(unsigned long arg)
+{
+	struct enq_blocked_stats stats = {
+		.hold_time_ns = atomic64_read(&hold_time_ns),
+		.wait_time_ns = atomic64_read(&wait_time_ns),
+		.nr_holds = atomic64_read(&nr_holds),
+		.nr_waits = atomic64_read(&nr_waits),
+	};
+
+	return copy_to_user((void __user *)arg, &stats, sizeof(stats)) ?
+		-EFAULT : 0;
+}
+
+static long enq_blocked_ioctl(struct file *file, unsigned int cmd,
+			      unsigned long arg)
+{
+	switch (cmd) {
+	case ENQ_BLOCKED_IOCTL_OWNER:
+		return run_owner();
+	case ENQ_BLOCKED_IOCTL_DONOR:
+		return run_donor();
+	case ENQ_BLOCKED_IOCTL_RESET_STATS:
+		reset_stats();
+		return 0;
+	case ENQ_BLOCKED_IOCTL_GET_STATS:
+		return get_stats(arg);
+	case ENQ_BLOCKED_IOCTL_PREP_ATTACH:
+		atomic_set(&attach_done, 0);
+		atomic_set(&attach_pending, 1);
+		return 0;
+	case ENQ_BLOCKED_IOCTL_ATTACH_DONE:
+		atomic_set(&attach_done, 1);
+		return 0;
+	case ENQ_BLOCKED_IOCTL_DONOR_STATE:
+		return get_donor_state();
+	case ENQ_BLOCKED_IOCTL_PROXY_SUPPORTED:
+		return IS_ENABLED(CONFIG_SCHED_PROXY_EXEC);
+	default:
+		return -EINVAL;
+	}
+}
+
+static const struct file_operations enq_blocked_fops = {
+	.owner			= THIS_MODULE,
+	.unlocked_ioctl		= enq_blocked_ioctl,
+};
+
+static struct miscdevice enq_blocked_device = {
+	.minor	= MISC_DYNAMIC_MINOR,
+	.name	= "scx_enq_blocked",
+	.fops	= &enq_blocked_fops,
+	.mode	= 0600,
+};
+
+module_misc_device(enq_blocked_device);
+MODULE_AUTHOR("Andrea Righi <arighi@nvidia.com>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("sched_ext blocked donor test module");
-- 
2.55.0

[PATCH 13/14] sched_ext: scx_qmap: Add proxy execution support

[Andrea Righi]

Add a -B option to opt scx_qmap into queueing mutex-blocked tasks for
proxy execution. Without the option, SCX_OPS_ENQ_BLOCKED remains clear
and mutex waiters block normally. With -B, blocked donors are passed to
qmap_enqueue() with SCX_ENQ_BLOCKED.

When scx_qmap receives a blocked donor, dispatch it directly to the
local DSQ of its current cid with a fresh slice and SCX_ENQ_PREEMPT.
This places the donor at the head of the DSQ and requests an immediate
reschedule, allowing the core proxy-exec path to run the mutex owner
using the donor's scheduling context as soon as the donor is selected.

The blocked policy is intentionally unfair and can strongly prioritize
tasks using contended mutexes, but scx_qmap is a demo scheduler and such
aggressive behavior makes proxy-exec support easy to observe. Count
these dispatch attempts in nr_enq_blocked and report their per-interval
delta.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 tools/sched_ext/scx_qmap.bpf.c | 34 ++++++++++++++++++++++++++++++++++
 tools/sched_ext/scx_qmap.c     | 13 ++++++++++---
 tools/sched_ext/scx_qmap.h     |  1 +
 3 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index aead17658573b..5ca1467c1325a 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -451,6 +451,40 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
 	 */
 	taskc->core_sched_seq = qa.core_sched_tail_seqs[idx]++;
 
+	/*
+	 * A blocked donor rejected by a local DSQ must not be dispatched back
+	 * to the same DSQ immediately. Move it to the global DSQ so another CPU
+	 * can pick it up and kick an idle cid to accelerate its consumption.
+	 */
+	if ((enq_flags & (SCX_ENQ_BLOCKED | SCX_ENQ_REENQ)) ==
+	    (SCX_ENQ_BLOCKED | SCX_ENQ_REENQ)) {
+		taskc->force_local = false;
+		scx_bpf_dsq_insert(p, SHARED_DSQ, 0, enq_flags);
+		cid = cmask_next_and2_set_wrap(&taskc->cpus_allowed,
+					       &qa.idle_cids.mask,
+					       &qa.self_cids.mask, 0);
+		if (cid < scx_bpf_nr_cids())
+			scx_bpf_kick_cid(cid, SCX_KICK_IDLE);
+		return;
+	}
+
+	/*
+	 * Insert a blocked mutex donor at the head of its current cid's local
+	 * DSQ with a fresh slice and %SCX_ENQ_PREEMPT, requesting an immediate
+	 * reschedule. Once selected, the core proxy-exec path can immediately
+	 * run the mutex owner using the donor's scheduling context.
+	 *
+	 * This policy is intentionally unfair and can strongly prioritize tasks
+	 * using contended mutexes; scx_qmap is a demonstration scheduler and
+	 * this behavior makes proxy-exec support easy to observe.
+	 */
+	if (enq_flags & SCX_ENQ_BLOCKED) {
+		__sync_fetch_and_add(&qa.nr_enq_blocked, 1);
+		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL_ON | scx_bpf_task_cid(p),
+				   slice_ns, enq_flags | SCX_ENQ_PREEMPT);
+		return;
+	}
+
 	/*
 	 * A node with children delegates most cids. A task of ours that can run
 	 * on none of our self cids (e.g. a per-NUMA kthread pinned to delegated
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index 27ffda1c519e8..8e943e8cb202d 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -46,7 +46,7 @@ const char help_fmt[] =
 "See the top-of-file comment in .bpf.c for the design.\n"
 "\n"
 "Usage: %s [-s SLICE_US] [-e COUNT] [-t COUNT] [-T COUNT] [-l COUNT] [-b COUNT]\n"
-"       [-N COUNT] [-P] [-M] [-H] [-c CG_PATH] [-d PID] [-D LEN] [-S] [-p] [-I]\n"
+"       [-N COUNT] [-P] [-M] [-H] [-c CG_PATH] [-d PID] [-D LEN] [-S] [-p] [-I] [-B]\n"
 "       [-F COUNT] [-i SEC] [-R MS] [-J MODE] [-v]\n"
 "\n"
 "  -s SLICE_US   Override slice duration\n"
@@ -65,6 +65,7 @@ const char help_fmt[] =
 "  -S            Suppress qmap-specific debug dump\n"
 "  -p            Switch only tasks on SCHED_EXT policy instead of all\n"
 "  -I            Turn on SCX_OPS_ALWAYS_ENQ_IMMED\n"
+"  -B            Turn on SCX_OPS_ENQ_BLOCKED\n"
 "  -F COUNT      IMMED stress: force every COUNT'th enqueue to a busy local DSQ (use with -I)\n"
 "  -C MODE       cid-override test (shuffle|bad-dup|bad-range|bad-mono)\n"
 "  -i SEC        Stats interval, seconds (default 5)\n"
@@ -105,6 +106,7 @@ struct hier_prev {
 	u64 nr_dsps[MAX_SUB_SCHEDS];
 	u64 nr_reenq_cap;
 	u64 nr_reenq_immed;
+	u64 nr_enq_blocked;
 	u64 nr_inject_attempts;
 };
 
@@ -187,13 +189,15 @@ static void print_hier(struct qmap_arena *qa, struct hier_prev *prev, u64 own_cg
 	}
 
 	format_cid_ranges(qa, CID_SHARED, ranges, sizeof(ranges));
-	printf("hier   : nsub=%llu excl=%u shared=%s rr=%s reenq cap/immed +%llu/+%llu inj=+%llu\n",
+	printf("hier   : nsub=%llu excl=%u shared=%s rr=%s reenq cap/immed +%llu/+%llu blocked=+%llu inj=+%llu\n",
 	       (unsigned long long)qa->nr_sub_scheds, qa->part.nr_excl, ranges, rr,
 	       (unsigned long long)(qa->nr_reenq_cap - prev->nr_reenq_cap),
 	       (unsigned long long)(qa->nr_reenq_immed - prev->nr_reenq_immed),
+	       (unsigned long long)(qa->nr_enq_blocked - prev->nr_enq_blocked),
 	       (unsigned long long)(qa->nr_inject_attempts - prev->nr_inject_attempts));
 	prev->nr_reenq_cap = qa->nr_reenq_cap;
 	prev->nr_reenq_immed = qa->nr_reenq_immed;
+	prev->nr_enq_blocked = qa->nr_enq_blocked;
 	prev->nr_inject_attempts = qa->nr_inject_attempts;
 
 	printf("hier   : %-4s %10s %4s %6s %8s  %s\n",
@@ -256,7 +260,7 @@ int main(int argc, char **argv)
 	skel->rodata->slice_ns = __COMPAT_ENUM_OR_ZERO("scx_public_consts", "SCX_SLICE_DFL");
 	skel->rodata->max_tasks = 16384;
 
-	while ((opt = getopt(argc, argv, "s:e:t:T:l:b:N:PMHc:d:D:SpIF:C:i:R:J:vh")) != -1) {
+	while ((opt = getopt(argc, argv, "s:e:t:T:l:b:N:PMHc:d:D:SpIBF:C:i:R:J:vh")) != -1) {
 		switch (opt) {
 		case 's':
 			skel->rodata->slice_ns = strtoull(optarg, NULL, 0) * 1000;
@@ -318,6 +322,9 @@ int main(int argc, char **argv)
 			skel->rodata->always_enq_immed = true;
 			skel->struct_ops.qmap_ops->flags |= SCX_OPS_ALWAYS_ENQ_IMMED;
 			break;
+		case 'B':
+			skel->struct_ops.qmap_ops->flags |= SCX_OPS_ENQ_BLOCKED;
+			break;
 		case 'F':
 			skel->rodata->immed_stress_nth = strtoul(optarg, NULL, 0);
 			break;
diff --git a/tools/sched_ext/scx_qmap.h b/tools/sched_ext/scx_qmap.h
index c42f7ef74b894..7f4d3b33e6133 100644
--- a/tools/sched_ext/scx_qmap.h
+++ b/tools/sched_ext/scx_qmap.h
@@ -174,6 +174,7 @@ struct qmap_arena {
 	/* bpf -> userspace: stats */
 	u64 nr_reenq_cap;		/* SCX_TASK_REENQ_CAP bounces */
 	u64 nr_reenq_immed;		/* SCX_TASK_REENQ_IMMED bounces */
+	u64 nr_enq_blocked;		/* SCX_ENQ_BLOCKED dispatches */
 	u64 nr_inject_attempts;		/* fault-injection: dispatches to an unheld cid */
 	u32 inject_mode;		/* fault-injection mode (QMAP_INJ_*) */
 };
-- 
2.55.0

[PATCH 14/14] sched: Allow enabling proxy exec with sched_ext

[Andrea Righi]

Now that sched_ext can handle proxy donors and BPF schedulers can opt in
to blocked-donor enqueueing with SCX_OPS_ENQ_BLOCKED, remove the
!SCHED_CLASS_EXT dependency from SCHED_PROXY_EXEC and allow both options
to be enabled together.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 init/Kconfig | 2 --
 1 file changed, 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 5230d4879b1c8..0817e62266e03 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -934,8 +934,6 @@ config SCHED_PROXY_EXEC
 	bool "Proxy Execution"
 	# Avoid some build failures w/ PREEMPT_RT until it can be fixed
 	depends on !PREEMPT_RT
-	# Need to investigate how to inform sched_ext of split contexts
-	depends on !SCHED_CLASS_EXT
 	# Not particularly useful until we get to multi-rq proxying
 	depends on EXPERT
 	help
-- 
2.55.0

Re: [PATCH 05/14] sched: Add sched_ext hooks for proxy execution

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The comment explaining the `should_block` argument for `try_to_block_task()` does not reflect the newly added `!scx_allow_proxy_exec(prev)` condition.
--

--- Patch [5]: [PATCH 05/14] sched: Add sched_ext hooks for proxy execution ---
commit 273c97aa298de804b9f343db4905abce55a2409e
Author: Andrea Righi <arighi@nvidia.com>

sched: Add sched_ext hooks for proxy execution

This commit introduces scx_allow_proxy_exec(), scx_proxy_donor_start(),
and scx_proxy_resolved() to observe proxy execution splits. It adds their
call sites in __schedule() as empty preparatory stubs for sched_ext.

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 66e9ba50f5639..c9d77b1e2fdc7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
[ ... ]
> @@ -7172,7 +7172,7 @@ static void __sched notrace __schedule(int sched_mode)
>  		 * task_is_blocked() will always be false).
>  		 */
>  		try_to_block_task(rq, prev, &prev_state,
> -				  !task_is_blocked(prev));
> +				  !task_is_blocked(prev) || !scx_allow_proxy_exec(prev));

[Severity: Low]
This isn't a bug, but should the comment directly above this call be updated
to reflect the new logic?

The comment explicitly states that only task_is_blocked() is passed as the
argument to keep mutex-blocked tasks on the runqueue, but this patch adds
scx_allow_proxy_exec(prev) to the condition without updating the explanation.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260725160513.57477-1-arighi@nvidia.com?part=5

Re: [PATCH 04/14] sched: Add prepare_switch() class callback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The `prepare_switch()` callback is invoked on the incoming `next_class`, but the task may never actually transition to that class if `SCHED_FLAG_KEEP_PARAMS` is set, leaving the prepared state orphaned and leaking resources.
--

--- Patch [4]: [PATCH 04/14] sched: Add prepare_switch() class callback ---
commit f8056ba7bb47e84cdf2c8a5193c3485b3b2d44d0
Author: Andrea Righi <arighi@nvidia.com>

sched: Add prepare_switch() class callback

This commit introduces a prepare_switch() callback for scheduling classes.
It allows an incoming class to prepare a task while the state is still
live, before the normal dequeue and class transition take place.

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 37c87783439f5..66e9ba50f5639 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
[ ... ]
> @@ -11221,7 +11221,9 @@ static inline void sched_mm_cid_fork(struct task_struct *t) { }
>  
>  static DEFINE_PER_CPU(struct sched_change_ctx, sched_change_ctx);
>  
> -struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags)
> +struct sched_change_ctx *
> +sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
> +		   unsigned int flags)
>  {
>  	struct sched_change_ctx *ctx = this_cpu_ptr(&sched_change_ctx);
>  	struct rq *rq = task_rq(p);
> @@ -11239,6 +11241,10 @@ struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int
>  		flags |= DEQUEUE_NOCLOCK;
>  	}
>  
> +	if ((flags & DEQUEUE_CLASS) && next_class != p->sched_class &&
> +	    next_class->prepare_switch)
> +		next_class->prepare_switch(rq, p);
> +
>  	if ((flags & DEQUEUE_CLASS) && p->sched_class->switching_from)
>  		p->sched_class->switching_from(rq, p);
>  
[ ... ]
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index b215b0ead9a60..bc32ce76ff4fe 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
[ ... ]
> @@ -678,7 +678,7 @@ int __sched_setscheduler(struct task_struct *p,
>  	if (prev_class != next_class)
>  		queue_flags |= DEQUEUE_CLASS;
>  
> -	scoped_guard (sched_change, p, queue_flags) {
> +	scoped_guard (sched_change, p, next_class, queue_flags) {
>  
>  		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
>  			__setscheduler_params(p, attr);

[Severity: Medium]
Will this leak resources if a user calls sched_setattr() with a new policy and
sets attr.sched_flags = SCHED_FLAG_KEEP_PARAMS?

In __sched_setscheduler(), if SCHED_FLAG_KEEP_PARAMS is set, the class
assignment p->sched_class = next_class is explicitly skipped.

However, because queue_flags includes DEQUEUE_CLASS, the newly modified
sched_change_begin() unconditionally invokes next_class->prepare_switch(rq, p)
for the incoming class.

Because the task never actually transitions to next_class, does this leave
the prepared state dangling and resources leaked since there won't be a
matching cleanup or transition?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260725160513.57477-1-arighi@nvidia.com?part=4

Re: [PATCH 02/14] sched: Make NOHZ CFS bandwidth checks follow proxy donor

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The newly added donor bandwidth check in `sched_can_stop_tick()` is bypassed by early returns when a FAIR donor delegates execution to a throttled RT task.
--

--- Patch [2]: [PATCH 02/14] sched: Make NOHZ CFS bandwidth checks follow proxy donor ---
commit 18e332090abc5adb7229eaf795a740846b882ae9
Author: Andrea Righi <arighi@nvidia.com>

sched: Make NOHZ CFS bandwidth checks follow proxy donor

This commit modifies sched_can_stop_tick() and sched_fair_update_stop_tick()
to check the CFS bandwidth constraints of the selected donor scheduling context
rather than the physical execution context. This prevents constrained FAIR
donors from evading bandwidth enforcement during proxy execution.

Link: https://lore.kernel.org/r/20260713164807.E5ED21F00A3A@smtp.kernel.org

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f38332274f8fe..ef0563970d412 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c

[ ... ]

> @@ -1462,14 +1459,17 @@ bool sched_can_stop_tick(struct rq *rq)
>  		return false;
>  
>  	/*
> -	 * If there is one task and it has CFS runtime bandwidth constraints
> -	 * and it's on the cpu now we don't want to stop the tick.
> +	 * If the selected scheduling context has CFS runtime bandwidth
> +	 * constraints, don't stop the tick. With proxy execution, rq->donor is
> +	 * the selected scheduling context while rq->curr is the task physically
> +	 * executing on its behalf.
> +	 *
>  	 * This check prevents clearing the bit if a newly enqueued task here is
> -	 * dequeued by migrating while the constrained task continues to run.
> +	 * dequeued by migrating while the constrained donor continues to run.
>  	 * E.g. going from 2->1 without going through pick_next_task().
>  	 */
> -	if (__need_bw_check(rq, rq->curr)) {
> -		if (cfs_task_bw_constrained(rq->curr))
> +	if (__need_bw_check(rq->donor)) {
> +		if (cfs_task_bw_constrained(rq->donor))
>  			return false;
>  	}

[Severity: High]
This is a pre-existing issue, but does this placement still allow a CFS
bandwidth-constrained FAIR task to evade its quota if it donates its context
to a throttled RT task?

If a FAIR task with CFS bandwidth constraints blocks on a mutex held by an
RT task, and the RT task exceeds its global RT runtime (becoming throttled),
the RT task's own context becomes ineligible. Proxy execution would then
select the FAIR donor's scheduling context (rq->donor), while the RT task
serves as the execution context (rq->curr).

During global RT throttling, dequeue_top_rt_rq() unsets rt_queued but does
not decrement rt_nr_running, leaving rq->rt.rt_nr_running > 0.

Because sched_can_stop_tick() has early checks for RT tasks (which can
return true when fifo_nr_running > 0), wouldn't the function return early
and completely bypass this newly added __need_bw_check(rq->donor) logic at
the bottom of the function?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260725160513.57477-1-arighi@nvidia.com?part=2