[PATCHSET sched_ext/for-7.3] sched_ext: Misc fixes

[PATCHSET sched_ext/for-7.3] sched_ext: Misc fixes

[Tejun Heo]

Hello,

Two unrelated small fixes.

 0001-sched_ext-Drop-unlocked-scx_rq_clock_invalidate-from.patch
 0002-sched_ext-Make-scx_bpf_events-read-the-calling-sched.patch

0001 drops the rq clock invalidation in the root disable path which was
performed without holding the rq lock and is redundant anyway. 0002 makes
scx_bpf_events() read the event counters of the scheduler associated with
the calling program instead of always reading the root scheduler's.

diffstat follows.

 kernel/sched/ext/ext.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

The patches are also applied to sched_ext/for-7.3.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git for-7.3

Thanks.

-- 
tejun

[PATCH 1/2] sched_ext: Drop unlocked scx_rq_clock_invalidate() from scx_root_disable()

[Tejun Heo]

scx_root_disable() invalidates each rq's clock before taking the rq lock.
scx_rq_clock_invalidate() is a plain read-modify-write of rq->scx.flags and
every other writer of the word runs under the rq lock, so the unlocked
update can race a concurrent flags update and lose one side's bits.

The invalidation is also redundant. The dl_server rebalancing right below
cycles each rq's lock and rq_unpin_lock() clears SCX_RQ_CLK_VALID on every
unlock, while __scx_enabled is turned off only later in the function, so the
clocks end up invalidated either way. Drop the call and note the
invalidation in the comment.

Fixes: 3a9910b5904d ("sched_ext: Implement scx_bpf_now()")
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Changwoo Min <changwoo@igalia.com>
---
 kernel/sched/ext/ext.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 2adf2bde0cb1..d301fe757193 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6476,12 +6476,13 @@ static void scx_root_disable(struct scx_sched *sch)
 	percpu_up_write(&scx_fork_rwsem);
 
 	/*
-	 * Invalidate all the rq clocks to prevent getting outdated
-	 * rq clocks from a previous scx scheduler.
+	 * Re-balance the dl_server bandwidth reservations: detach ext_server
+	 * (no more sched_ext tasks) and reinstate fair_server if it was
+	 * previously detached because we were running in full mode.
 	 *
-	 * Also re-balance the dl_server bandwidth reservations: detach
-	 * ext_server (no more sched_ext tasks) and reinstate fair_server if it
-	 * was previously detached because we were running in full mode.
+	 * The rq lock cycle also invalidates each rq's clock (rq_unpin_lock()
+	 * clears SCX_RQ_CLK_VALID on unlock), preventing the next enable from
+	 * seeing outdated rq clocks from this scheduler.
 	 *
 	 * Unlike the enable path, this runs on a recovery path that cannot
 	 * fail, so we use dl_server_swap_bw() to atomically free ext_server's
@@ -6494,8 +6495,6 @@ static void scx_root_disable(struct scx_sched *sch)
 	for_each_possible_cpu(cpu) {
 		struct rq *rq = cpu_rq(cpu);
 
-		scx_rq_clock_invalidate(rq);
-
 		scoped_guard(rq_lock_irqsave, rq) {
 			update_rq_clock(rq);
 			if (was_switched_all) {
-- 
2.55.0

[PATCH v2 1/2] sched_ext: Drop unlocked scx_rq_clock_invalidate() from scx_root_disable()

[Tejun Heo]

scx_root_disable() invalidates each rq's clock before taking the rq lock.
scx_rq_clock_invalidate() is a plain read-modify-write of rq->scx.flags and
every other writer of the word runs under the rq lock, so the unlocked
update can race a concurrent flags update and lose one side's bits.

The invalidation doesn't matter in the first place. The cached clock is read
only by scx_bpf_now() from a loaded scheduler's BPF programs, nothing can
re-validate the clock while sched_ext is disabled as scx_rq_clock_update()
is gated on scx_enabled() too, and the usual rq lock cycles under the next
scheduler refresh or invalidate it before it's practically observable. Drop
the invalidation instead of fixing the locking.

v2: Description and comment updated - the invalidation is unnecessary rather
    than subsumed by the rq lock cycle below.

Fixes: 3a9910b5904d ("sched_ext: Implement scx_bpf_now()")
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Changwoo Min <changwoo@igalia.com>
---
 kernel/sched/ext/ext.c |   11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6367,12 +6367,9 @@ static void scx_root_disable(struct scx_
 	percpu_up_write(&scx_fork_rwsem);
 
 	/*
-	 * Invalidate all the rq clocks to prevent getting outdated
-	 * rq clocks from a previous scx scheduler.
-	 *
-	 * Also re-balance the dl_server bandwidth reservations: detach
-	 * ext_server (no more sched_ext tasks) and reinstate fair_server if it
-	 * was previously detached because we were running in full mode.
+	 * Re-balance the dl_server bandwidth reservations: detach ext_server
+	 * (no more sched_ext tasks) and reinstate fair_server if it was
+	 * previously detached because we were running in full mode.
 	 *
 	 * Unlike the enable path, this runs on a recovery path that cannot
 	 * fail, so we use dl_server_swap_bw() to atomically free ext_server's
@@ -6385,8 +6382,6 @@ static void scx_root_disable(struct scx_
 	for_each_possible_cpu(cpu) {
 		struct rq *rq = cpu_rq(cpu);
 
-		scx_rq_clock_invalidate(rq);
-
 		scoped_guard(rq_lock_irqsave, rq) {
 			update_rq_clock(rq);
 			if (was_switched_all) {

[PATCH 2/2] sched_ext: Make scx_bpf_events() read the calling scheduler's counters

[Tejun Heo]

scx_bpf_events() always reads the root scheduler's event counters, so a
sub-scheduler program querying its own events silently gets the root's
instead and has no BPF-visible way to read its own (the per-scheduler sysfs
"events" file is the only interface). Resolve the scheduler from the calling
program with scx_prog_sched(). Unassociated programs follow the usual
scx_prog_sched() resolution: the root scheduler under a pre-sub-attach
compat root and zeroed counters otherwise.

Also fix up the malformed comment into proper kerneldoc.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/ext.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index d301fe757193..b9060c318c3c 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -10608,19 +10608,23 @@ static void scx_read_events(struct scx_sched *sch, struct scx_event_stats *event
 	}
 }
 
-/*
- * scx_bpf_events - Get a system-wide event counter to
+/**
+ * scx_bpf_events - Read the event counters of the calling scheduler
  * @events: output buffer from a BPF program
- * @events__sz: @events len, must end in '__sz'' for the verifier
+ * @events__sz: @events len, must end in '__sz' for the verifier
+ * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
+ *
+ * Read the event counters of the scheduler associated with the calling program.
+ * @events is zeroed when no scheduler can be resolved.
  */
-__bpf_kfunc void scx_bpf_events(struct scx_event_stats *events,
-				size_t events__sz)
+__bpf_kfunc void scx_bpf_events(struct scx_event_stats *events, size_t events__sz,
+				const struct bpf_prog_aux *aux)
 {
 	struct scx_sched *sch;
 	struct scx_event_stats e_sys;
 
 	rcu_read_lock();
-	sch = rcu_dereference(scx_root);
+	sch = scx_prog_sched(aux);
 	if (sch)
 		scx_read_events(sch, &e_sys);
 	else
@@ -10743,7 +10747,7 @@ BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PRO
 BTF_ID_FLAGS(func, scx_bpf_cid_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
 BTF_ID_FLAGS(func, scx_bpf_tid_to_task, KF_RET_NULL | KF_RCU_PROTECTED)
 BTF_ID_FLAGS(func, scx_bpf_now)
-BTF_ID_FLAGS(func, scx_bpf_events)
+BTF_ID_FLAGS(func, scx_bpf_events, KF_IMPLICIT_ARGS)
 #ifdef CONFIG_CGROUP_SCHED
 BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_IMPLICIT_ARGS | KF_RCU | KF_ACQUIRE)
 #endif
-- 
2.55.0

Re: [PATCHSET sched_ext/for-7.3] sched_ext: Misc fixes

[Tejun Heo]

Applied to sched_ext/for-7.3 with the v2 description for the first patch.

Thanks.

-- 
tejun