[PATCHSET sched_ext/for-7.3] sched_ext: Sub-scheduler follow-ups

[PATCHSET sched_ext/for-7.3] sched_ext: Sub-scheduler follow-ups

[Tejun Heo]

Hello,

Follow-ups to the sub-scheduler capability series, addressing two things
Andrea raised while reviewing v5[1]: sched teardown drained queued ecaps
syncs with an msleep() poll loop, and with CONFIG_EXT_SUB_SCHED=y hot paths
keep paying for sub-sched bookkeeping even when no sub-scheduler exists.

Patch 0001 replaces the teardown poll loop with direct removal from the
llist. Patches 0002-0003 are prep and 0004 adds the scx_has_subs static key
and gates the sub-sched portions of hot paths on it.

[1] https://lore.kernel.org/r/20260709225041.1695495-1-tj@kernel.org

Based on sched_ext/for-7.3 (35f9cbbacb67).

This patchset contains the following 4 patches.

 0001 sched_ext: Remove queued ecaps syncs directly on sched teardown
 0002 sched_ext: Move scx_dispatch_sched() from sub.h to internal.h
 0003 sched_ext: Gate sub_dispatch_prev with CONFIG_EXT_SUB_SCHED
 0004 sched_ext: Add the scx_has_subs static key and gate sub-sched hot paths

The patchset is also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-sub-followups

diffstat follows. Thanks.

 kernel/sched/ext/ext.c      |   6 +-
 kernel/sched/ext/idle.c     |  10 ++++
 kernel/sched/ext/internal.h | 133 +++++++++++++++++++++++++++++++++++++++++++-
 kernel/sched/ext/sub.c      |  88 ++++++++++++++++++++---------
 kernel/sched/ext/sub.h      | 124 +++++------------------------------------
 kernel/sched/sched.h        |   3 +-
 6 files changed, 223 insertions(+), 141 deletions(-)

--
tejun

[PATCH 1/4] sched_ext: Remove queued ecaps syncs directly on sched teardown

[Tejun Heo]

scx_discard_ecaps_to_sync() waited for balance_one() to consume a dying
sched's queued ecaps sync, polling with resched_cpu() + msleep(). The wait
is unbounded - the ext dl_server forces picks through sustained fair or RT
load only while ext tasks are queued, so an ext-idle cpu monopolized by a
higher class can stall the teardown indefinitely.

Remove the node directly instead: take all queued nodes, drop the dying
sched's and resplice the rest. Consumption runs under the rq lock and batch
nodes read as on-list throughout, so the producer-side dedup stays correct.

A node that an in-flight scx_process_sync_ecaps() batch holds across a
dispatch-induced rq unlock still needs a wait, but one bounded by that batch
completing rather than by a future balance.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/ext.c |  2 +-
 kernel/sched/ext/sub.c | 67 ++++++++++++++++++++++++++----------------
 2 files changed, 43 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index f8f8597b6f78..428f2bfb2cda 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4877,7 +4877,7 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
 		 */
 		WARN_ON_ONCE(!list_empty(&pcpu->deferred_reenq_local.node));
 
-		/* retire the queued ecaps syncs so the pcpu can be freed */
+		/* remove the queued ecaps sync so the pcpu can be freed */
 		scx_discard_ecaps_to_sync(cpu, pcpu);
 
 		/*
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 68ab3675337f..198063a78a3a 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -13,7 +13,6 @@
  * Copyright (c) 2026 Meta Platforms, Inc. and affiliates.
  * Copyright (c) 2026 Tejun Heo <tj@kernel.org>
  */
-#include <linux/delay.h>
 #include <linux/rhashtable.h>
 #include "internal.h"
 #include "cid.h"
@@ -537,7 +536,12 @@ void scx_process_sync_ecaps(struct rq *rq, struct task_struct *prev)
 		gained = ecaps & ~old;
 		lost_all |= lost;
 
-		/* tell the sched its effective caps on this cid changed */
+		/*
+		 * Tell the sched its effective caps on this cid changed. The
+		 * invocation is equivalent to the dispatch path and may drop
+		 * and re-acquire the rq lock temporarily while the rest of
+		 * @batch is held privately, see scx_discard_ecaps_to_sync().
+		 */
 		if (ecaps != pcpu->reported_ecaps &&
 		    SCX_HAS_OP(pcpu->sch, sub_ecaps_updated) &&
 		    !scx_bypassing(pcpu->sch, cpu)) {
@@ -661,38 +665,51 @@ void scx_offline_ecaps(struct rq *rq)
 }
 
 /*
- * @pcpu's sched was unhashed before the grace period, so nothing new queues.
- * Flush its pending sync so the pcpu can be freed. If the cpu is online and
- * scx is enabled, drain via balance_one(). Otherwise, discard under the rq
- * lock.
+ * @pcpu's sched was unhashed before the grace period, so nothing re-queues its
+ * sync node. Remove the node from @rq's pending list so the pcpu can be freed.
  */
 void scx_discard_ecaps_to_sync(s32 cpu, struct scx_sched_pcpu *pcpu)
 {
 	struct rq *rq = cpu_rq(cpu);
+	struct llist_node *head = NULL, *tail = NULL;
+	struct llist_node *pos, *tmp;
+
+	/*
+	 * llist can't unlink a single node. Take all queued nodes, drop @pcpu's
+	 * and resplice the rest. Nodes in the taken batch read as on-list
+	 * throughout, so queue_sync_ecaps() stays correct.
+	 */
+	if (llist_on_list(&pcpu->ecaps_to_sync_node)) {
+		scoped_guard (rq_lock_irqsave, rq) {
+			llist_for_each_safe(pos, tmp, llist_del_all(&rq->scx.ecaps_to_sync)) {
+				if (pos == &pcpu->ecaps_to_sync_node) {
+					init_llist_node(pos);
+				} else {
+					pos->next = head;
+					head = pos;
+					if (!tail)
+						tail = pos;
+				}
+			}
+			if (head)
+				llist_add_batch(head, tail, &rq->scx.ecaps_to_sync);
+		}
+	}
 
+	/*
+	 * An in-flight scx_process_sync_ecaps() batch may still hold the node
+	 * privately across dispatch-induced rq unlocks, reading as on-list.
+	 *
+	 * Because a bypassing sched gets no op call, init_llist_node() and all
+	 * @pcpu accesses share one contiguous lock hold, off-list under the rq
+	 * lock means @pcpu won't be accessed again.
+	 */
 	while (true) {
 		scoped_guard (rq_lock_irqsave, rq) {
-			/*
-			 * scx_process_sync_ecaps() takes the node off the list
-			 * before it is done accessing @pcpu but does all of it
-			 * under the rq lock. Off-list observed under the rq
-			 * lock guarantees that the sync is complete.
-			 */
 			if (!llist_on_list(&pcpu->ecaps_to_sync_node))
 				return;
-			/*
-			 * Discard only when the cpu is truly down. cpu_active()
-			 * is already set when scx_online_ecaps() queues an online
-			 * resync while SCX_RQ_ONLINE is not - so test cpu_active(),
-			 * or that resync would be dropped.
-			 */
-			if (!scx_enabled() || !cpu_active(cpu)) {
-				discard_queued_syncs(rq);
-				return;
-			}
 		}
-		resched_cpu(cpu);
-		msleep(1);
+		cpu_relax();
 	}
 }
 
@@ -704,7 +721,7 @@ void scx_discard_ecaps_to_sync(s32 cpu, struct scx_sched_pcpu *pcpu)
  * still-open link fd defers it) and can leave queued ecaps syncs behind.
  * Processing them would decode the dead sched's pshards with the current cid
  * layout. Discard them instead. The backing scx_sched_pcpu's are still
- * allocated as the free path drains ecaps_to_sync_node before freeing.
+ * allocated as the free path removes ecaps_to_sync_node before freeing.
  */
 void scx_discard_stale_ecaps_syncs(void)
 {
-- 
2.55.0

[PATCH 2/4] sched_ext: Move scx_dispatch_sched() from sub.h to internal.h

[Tejun Heo]

scx_dispatch_sched() is common dispatch machinery, not sub-scheduler support
code, and looks out of place in sub.h. Move it to internal.h. cid.h is
included near the tail for scx_cpu_arg() as it depends on internal.h
declarations. Pure code move, no functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/internal.h | 111 ++++++++++++++++++++++++++++++++++++
 kernel/sched/ext/sub.h      | 109 -----------------------------------
 2 files changed, 111 insertions(+), 109 deletions(-)

diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index d7a1d6a14ebf..88b2a9b5d3ab 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -2289,4 +2289,115 @@ static inline struct scx_sched *scx_prog_sched(const struct bpf_prog_aux *aux)
 static inline struct scx_sched *scx_parent(struct scx_sched *sch) { return NULL; }
 #endif	/* CONFIG_EXT_SUB_SCHED */
 
+#include "cid.h"
+
+/*
+ * One user of this function is scx_bpf_dispatch() which can be called
+ * recursively as sub-sched dispatches nest. Always inline to reduce stack usage
+ * from the call frame.
+ */
+static __always_inline bool
+scx_dispatch_sched(struct scx_sched *sch, struct rq *rq,
+		   struct task_struct *prev, bool nested)
+{
+	struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;
+	int nr_loops = SCX_DSP_MAX_LOOPS;
+	s32 cpu = cpu_of(rq);
+	bool prev_on_sch = (prev->sched_class == &ext_sched_class) &&
+		scx_task_on_sched(sch, prev);
+
+	if (scx_consume_global_dsq(sch, rq))
+		return true;
+
+	if (scx_bypass_dsp_enabled(sch)) {
+		/* if @sch is bypassing, only the bypass DSQs are active */
+		if (scx_bypassing(sch, cpu))
+			return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0);
+
+#ifdef CONFIG_EXT_SUB_SCHED
+		/*
+		 * If @sch isn't bypassing but its children are, @sch is
+		 * responsible for making forward progress for both its own
+		 * tasks that aren't bypassing and the bypassing descendants'
+		 * tasks. The following implements a simple built-in behavior -
+		 * let each CPU try to run the bypass DSQ every Nth time.
+		 *
+		 * Later, if necessary, we can add an ops flag to suppress the
+		 * auto-consumption and a kfunc to consume the bypass DSQ and,
+		 * so that the BPF scheduler can fully control scheduling of
+		 * bypassed tasks.
+		 */
+		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);
+
+		if (!(pcpu->bypass_host_seq++ % SCX_BYPASS_HOST_NTH) &&
+		    scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0)) {
+			__scx_add_event(sch, SCX_EV_SUB_BYPASS_DISPATCH, 1);
+			return true;
+		}
+#endif	/* CONFIG_EXT_SUB_SCHED */
+	}
+
+	if (unlikely(!SCX_HAS_OP(sch, dispatch)) || !scx_rq_online(rq))
+		return false;
+
+	dspc->rq = rq;
+
+	/*
+	 * The dispatch loop. Because scx_flush_dispatch_buf() may drop the rq
+	 * lock, the local DSQ might still end up empty after a successful
+	 * ops.dispatch(). If the local DSQ is empty even after ops.dispatch()
+	 * produced some tasks, retry. The BPF scheduler may depend on this
+	 * looping behavior to simplify its implementation.
+	 */
+	do {
+		dspc->nr_tasks = 0;
+
+		if (nested) {
+			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
+				    prev_on_sch ? prev : NULL);
+		} else {
+			/* stash @prev so that nested invocations can access it */
+			rq->scx.sub_dispatch_prev = prev;
+			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
+				    prev_on_sch ? prev : NULL);
+			rq->scx.sub_dispatch_prev = NULL;
+		}
+
+		scx_flush_dispatch_buf(sch, rq);
+
+		if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice) {
+			rq->scx.flags |= SCX_RQ_BAL_KEEP;
+			return true;
+		}
+		if (rq->scx.local_dsq.nr)
+			return true;
+		if (scx_consume_global_dsq(sch, rq))
+			return true;
+
+		/*
+		 * ops.dispatch() can trap us in this loop by repeatedly
+		 * dispatching ineligible tasks. Break out once in a while to
+		 * allow the watchdog to run. As IRQ can't be enabled in
+		 * balance(), we want to complete this scheduling cycle and then
+		 * start a new one. IOW, we want to call resched_curr() on the
+		 * next, most likely idle, task, not the current one. Use
+		 * __scx_bpf_kick_cpu() for deferred kicking.
+		 */
+		if (unlikely(!--nr_loops)) {
+			scx_kick_cpu(sch, cpu, 0);
+			break;
+		}
+	} while (dspc->nr_tasks);
+
+	/*
+	 * Prevent the CPU from going idle while bypassed descendants have tasks
+	 * queued. Without this fallback, bypassed tasks could stall if the host
+	 * scheduler's ops.dispatch() doesn't yield any tasks.
+	 */
+	if (scx_bypass_dsp_enabled(sch))
+		return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0);
+
+	return false;
+}
+
 #endif /* _KERNEL_SCHED_EXT_INTERNAL_H */
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index f72c18a5972a..d06bf24aee83 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -172,113 +172,4 @@ static inline bool scx_task_can_stay_on_cpu(struct rq *rq, struct task_struct *p
 
 #endif	/* CONFIG_EXT_SUB_SCHED */
 
-/*
- * One user of this function is scx_bpf_dispatch() which can be called
- * recursively as sub-sched dispatches nest. Always inline to reduce stack usage
- * from the call frame.
- */
-static __always_inline bool
-scx_dispatch_sched(struct scx_sched *sch, struct rq *rq,
-		   struct task_struct *prev, bool nested)
-{
-	struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;
-	int nr_loops = SCX_DSP_MAX_LOOPS;
-	s32 cpu = cpu_of(rq);
-	bool prev_on_sch = (prev->sched_class == &ext_sched_class) &&
-		scx_task_on_sched(sch, prev);
-
-	if (scx_consume_global_dsq(sch, rq))
-		return true;
-
-	if (scx_bypass_dsp_enabled(sch)) {
-		/* if @sch is bypassing, only the bypass DSQs are active */
-		if (scx_bypassing(sch, cpu))
-			return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0);
-
-#ifdef CONFIG_EXT_SUB_SCHED
-		/*
-		 * If @sch isn't bypassing but its children are, @sch is
-		 * responsible for making forward progress for both its own
-		 * tasks that aren't bypassing and the bypassing descendants'
-		 * tasks. The following implements a simple built-in behavior -
-		 * let each CPU try to run the bypass DSQ every Nth time.
-		 *
-		 * Later, if necessary, we can add an ops flag to suppress the
-		 * auto-consumption and a kfunc to consume the bypass DSQ and,
-		 * so that the BPF scheduler can fully control scheduling of
-		 * bypassed tasks.
-		 */
-		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);
-
-		if (!(pcpu->bypass_host_seq++ % SCX_BYPASS_HOST_NTH) &&
-		    scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0)) {
-			__scx_add_event(sch, SCX_EV_SUB_BYPASS_DISPATCH, 1);
-			return true;
-		}
-#endif	/* CONFIG_EXT_SUB_SCHED */
-	}
-
-	if (unlikely(!SCX_HAS_OP(sch, dispatch)) || !scx_rq_online(rq))
-		return false;
-
-	dspc->rq = rq;
-
-	/*
-	 * The dispatch loop. Because scx_flush_dispatch_buf() may drop the rq
-	 * lock, the local DSQ might still end up empty after a successful
-	 * ops.dispatch(). If the local DSQ is empty even after ops.dispatch()
-	 * produced some tasks, retry. The BPF scheduler may depend on this
-	 * looping behavior to simplify its implementation.
-	 */
-	do {
-		dspc->nr_tasks = 0;
-
-		if (nested) {
-			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
-				    prev_on_sch ? prev : NULL);
-		} else {
-			/* stash @prev so that nested invocations can access it */
-			rq->scx.sub_dispatch_prev = prev;
-			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
-				    prev_on_sch ? prev : NULL);
-			rq->scx.sub_dispatch_prev = NULL;
-		}
-
-		scx_flush_dispatch_buf(sch, rq);
-
-		if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice) {
-			rq->scx.flags |= SCX_RQ_BAL_KEEP;
-			return true;
-		}
-		if (rq->scx.local_dsq.nr)
-			return true;
-		if (scx_consume_global_dsq(sch, rq))
-			return true;
-
-		/*
-		 * ops.dispatch() can trap us in this loop by repeatedly
-		 * dispatching ineligible tasks. Break out once in a while to
-		 * allow the watchdog to run. As IRQ can't be enabled in
-		 * balance(), we want to complete this scheduling cycle and then
-		 * start a new one. IOW, we want to call resched_curr() on the
-		 * next, most likely idle, task, not the current one. Use
-		 * __scx_bpf_kick_cpu() for deferred kicking.
-		 */
-		if (unlikely(!--nr_loops)) {
-			scx_kick_cpu(sch, cpu, 0);
-			break;
-		}
-	} while (dspc->nr_tasks);
-
-	/*
-	 * Prevent the CPU from going idle while bypassed descendants have tasks
-	 * queued. Without this fallback, bypassed tasks could stall if the host
-	 * scheduler's ops.dispatch() doesn't yield any tasks.
-	 */
-	if (scx_bypass_dsp_enabled(sch))
-		return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0);
-
-	return false;
-}
-
 #endif /* _KERNEL_SCHED_EXT_SUB_H */
-- 
2.55.0

[PATCH 3/4] sched_ext: Gate sub_dispatch_prev with CONFIG_EXT_SUB_SCHED

[Tejun Heo]

rq->scx.sub_dispatch_prev is sub-sched-only but was left unconditional. Move
it into the CONFIG_EXT_SUB_SCHED block next to ecaps_to_sync and gate its
updates.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/internal.h | 19 +++++++++++--------
 kernel/sched/sched.h        |  3 +--
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index 88b2a9b5d3ab..8c940be9278d 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -2352,16 +2352,19 @@ scx_dispatch_sched(struct scx_sched *sch, struct rq *rq,
 	do {
 		dspc->nr_tasks = 0;
 
-		if (nested) {
-			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
-				    prev_on_sch ? prev : NULL);
-		} else {
-			/* stash @prev so that nested invocations can access it */
+#ifdef CONFIG_EXT_SUB_SCHED
+		/* stash @prev so that nested invocations can access it */
+		if (!nested)
 			rq->scx.sub_dispatch_prev = prev;
-			SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
-				    prev_on_sch ? prev : NULL);
+#endif
+
+		SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu),
+			    prev_on_sch ? prev : NULL);
+
+#ifdef CONFIG_EXT_SUB_SCHED
+		if (!nested)
 			rq->scx.sub_dispatch_prev = NULL;
-		}
+#endif
 
 		scx_flush_dispatch_buf(sch, rq);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 54cff94556c0..64d79e9efc3d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -814,6 +814,7 @@ struct scx_rq {
 	u64			clock;			/* current per-rq clock -- see scx_bpf_now() */
 #ifdef CONFIG_EXT_SUB_SCHED
 	struct llist_head	ecaps_to_sync;		/* pending ecaps syncs */
+	struct task_struct	*sub_dispatch_prev;
 #endif
 	cpumask_var_t		cpus_to_sync;
 	bool			kick_sync_pending;
@@ -821,8 +822,6 @@ struct scx_rq {
 
 	struct list_head	sched_pcpus_to_kick;	/* see kick_cpus_irq_workfn() */
 
-	struct task_struct	*sub_dispatch_prev;
-
 	raw_spinlock_t		deferred_reenq_lock;
 	u64			deferred_reenq_locals_seq;
 	struct list_head	deferred_reenq_locals;	/* scheds requesting reenq of local DSQ */
-- 
2.55.0

[PATCH 4/4] sched_ext: Add the scx_has_subs static key and gate sub-sched hot paths

[Tejun Heo]

With CONFIG_EXT_SUB_SCHED=y but no sub-scheduler attached - the common case
- hot paths still pay for sub-sched bookkeeping. Gate it behind
__scx_has_subs, a static key counting live sub-schedulers, so that a
root-only system stops paying.

Most conversions are simple skip-if-no-sub tests. scx_idle_notify() is
special - it's a hierarchy walk, so give it a fast path which notifies the
root directly using the same tests as the walk. A pending
SCX_RQ_SUB_IDLE_RENOTIFY can be ignored as no sub can be owed one and the
caller clears the flag either way.

Suggested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/ext.c      |  4 ++++
 kernel/sched/ext/idle.c     | 10 ++++++++++
 kernel/sched/ext/internal.h | 19 +++++++++++++++++--
 kernel/sched/ext/sub.c      | 21 +++++++++++++++++++--
 kernel/sched/ext/sub.h      | 15 +++++++++++++++
 5 files changed, 65 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 428f2bfb2cda..baf5a3daf7db 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4919,6 +4919,10 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
 	scx_arena_pool_destroy(sch);
 	if (sch->arena_map)
 		bpf_map_put(sch->arena_map);
+
+	/* @sch is completely inactive by now */
+	scx_dec_has_subs(sch);
+
 	kfree(sch);
 }
 
diff --git a/kernel/sched/ext/idle.c b/kernel/sched/ext/idle.c
index 16ebe3ab8647..d08166de03d8 100644
--- a/kernel/sched/ext/idle.c
+++ b/kernel/sched/ext/idle.c
@@ -746,6 +746,16 @@ static void scx_idle_notify(struct rq *rq, bool idle, bool do_notify, bool root_
 
 	lockdep_assert_rq_held(rq);
 
+	/* with no sub-sched, only the root can be owed a notification */
+	if (!scx_has_subs()) {
+		struct scx_sched *sch = scx_root;
+
+		if ((do_notify || root_renotify) &&
+		    SCX_HAS_OP(sch, update_idle) && !scx_bypassing(sch, cpu))
+			SCX_CALL_OP(sch, update_idle, rq, cid, idle);
+		return;
+	}
+
 	pos = scx_next_descendant_pre(NULL, scx_root);
 	while (pos) {
 		bool forced = false;
diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index 8c940be9278d..e69aa04c6739 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -2110,7 +2110,7 @@ do {										\
  */
 #define SCX_CALL_OP_TASK(sch, op, locked_rq, task, args...)			\
 do {										\
-	WARN_ON_ONCE((sch) != scx_task_sched_rcu(task));			\
+	WARN_ON_ONCE(scx_has_subs() && (sch) != scx_task_sched_rcu(task));	\
 	__SCX_CALL_OP_TASK((sch), ops, op, locked_rq, task, ##args);		\
 } while (0)
 
@@ -2125,7 +2125,7 @@ do {										\
 #define SCX_CALL_OP_TASK_RET(sch, op, locked_rq, task, args...)			\
 ({										\
 	__typeof__((sch)->ops.op(task, ##args)) __ret;				\
-	WARN_ON_ONCE((sch) != scx_task_sched_rcu(task));			\
+	WARN_ON_ONCE(scx_has_subs() && (sch) != scx_task_sched_rcu(task));	\
 	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
 	current->scx.kf_tasks[0] = task;					\
 	__ret = SCX_CALL_OP_RET((sch), op, locked_rq, task, ##args);		\
@@ -2165,6 +2165,19 @@ static inline bool scx_bypassing(struct scx_sched *sch, s32 cpu)
 }
 
 #ifdef CONFIG_EXT_SUB_SCHED
+DECLARE_STATIC_KEY_FALSE(__scx_has_subs);
+
+/**
+ * scx_has_subs - Whether any sub-scheduler exists
+ *
+ * Gates the sub-sched portions of hot paths so that a root-only system doesn't
+ * pay for them. See scx_sub_enable_workfn() and scx_sched_free_rcu_work().
+ */
+static inline bool scx_has_subs(void)
+{
+	return static_branch_unlikely(&__scx_has_subs);
+}
+
 /**
  * scx_task_sched - Find scx_sched scheduling a task
  * @p: task of interest
@@ -2259,6 +2272,8 @@ static inline struct scx_sched *scx_parent(struct scx_sched *sch)
 		return NULL;
 }
 #else	/* CONFIG_EXT_SUB_SCHED */
+static inline bool scx_has_subs(void) { return false; }
+
 static inline struct scx_sched *scx_task_sched(const struct task_struct *p)
 {
 	return rcu_dereference_protected(scx_root,
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 198063a78a3a..d92fae5888ac 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -21,6 +21,12 @@
 
 #ifdef CONFIG_EXT_SUB_SCHED
 
+/*
+ * On while any sub-scheduler exists so that a root-only system doesn't pay for
+ * the sub-sched portions of hot paths. See scx_has_subs().
+ */
+DEFINE_STATIC_KEY_FALSE(__scx_has_subs);
+
 /**
  * scx_skip_subtree_pre - Skip @pos's subtree in a pre-order walk
  * @pos: current position
@@ -235,6 +241,9 @@ void scx_init_root_caps(struct scx_sched *sch)
 struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq *rq,
 					       struct task_struct *p, u64 *enq_flags)
 {
+	if (!scx_has_subs())
+		return &rq->scx.local_dsq;
+
 	s32 cid = __scx_cpu_to_cid(cpu_of(rq));
 	struct scx_sched *asch = rq->scx.remote_activate_sch ?: sch;
 	u64 needed = scx_caps_for_enq(*enq_flags);
@@ -313,7 +322,7 @@ void scx_reenq_reject(struct rq *rq)
 
 	lockdep_assert_rq_held(rq);
 
-	if (list_empty(&rq->scx.reject_dsq.list))
+	if (!scx_has_subs() || list_empty(&rq->scx.reject_dsq.list))
 		return;
 
 	/*
@@ -496,7 +505,7 @@ void scx_process_sync_ecaps(struct rq *rq, struct task_struct *prev)
 
 	lockdep_assert_rq_held(rq);
 
-	if (likely(llist_empty(&rq->scx.ecaps_to_sync)))
+	if (!scx_has_subs() || likely(llist_empty(&rq->scx.ecaps_to_sync)))
 		return;
 
 	/*
@@ -1015,10 +1024,18 @@ void scx_sub_enable_workfn(struct kthread_work *work)
 	kobject_get(&parent->kobj);
 	raw_spin_unlock_irq(&scx_sched_lock);
 
+	/*
+	 * Flip the hot-path gates before ops->priv is published - the sub's
+	 * programs can e.g. kick cpus from that point on. The matching dec is
+	 * at the end of scx_sched_free_rcu_work().
+	 */
+	static_branch_inc(&__scx_has_subs);
+
 	/* scx_alloc_and_add_sched() consumes @cgrp whether it succeeds or not */
 	sch = scx_alloc_and_add_sched(cmd, cgrp, parent);
 	kobject_put(&parent->kobj);
 	if (IS_ERR(sch)) {
+		static_branch_dec(&__scx_has_subs);
 		ret = PTR_ERR(sch);
 		goto out_unlock;
 	}
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index d06bf24aee83..d5f377d35352 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -45,6 +45,13 @@ static inline const char *sch_cgrp_path(struct scx_sched *sch)
 	return sch->cgrp_path;
 }
 
+/* a dying sub's hot-path influence ends in scx_sched_free_rcu_work() */
+static inline void scx_dec_has_subs(struct scx_sched *sch)
+{
+	if (sch->level)
+		static_branch_dec(&__scx_has_subs);
+}
+
 #else	/* CONFIG_EXT_SUB_SCHED */
 
 static inline struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos, struct scx_sched *root) { return pos ? NULL : root; }
@@ -67,6 +74,7 @@ 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 */
 
@@ -97,6 +105,10 @@ static inline u64 scx_missing_caps(struct scx_sched *sch, s32 cpu, u64 needed)
 {
 	u64 ecaps;
 
+	/* no sub-scheds, no missing caps */
+	if (!scx_has_subs())
+		return 0;
+
 	/* root holds every cap on every cpu */
 	if (!sch->level)
 		return 0;
@@ -157,6 +169,9 @@ static inline u64 scx_caps_implied(u64 cap)
 /* may @p keep running on @rq's cpu? requires baseline cpu access */
 static inline bool scx_task_can_stay_on_cpu(struct rq *rq, struct task_struct *p)
 {
+	if (!scx_has_subs())
+		return true;
+
 	/* a migration-disabled task is let in without caps, keep it likewise */
 	if (unlikely(is_migration_disabled(p)))
 		return true;
-- 
2.55.0

Re: [PATCH 2/4] sched_ext: Move scx_dispatch_sched() from sub.h to internal.h

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Moving `scx_dispatch_sched()` to `internal.h` and including `cid.h` at the end creates a circular header dependency. `cid.h` includes `internal.h` at the top. When `cid.h` is included first by a C file, it triggers the inclusion of `internal.h`. The `#include "cid.h"` at the end of `internal.h` is skipped by the include guard. Consequently, `scx_dispatch_sched()` is parsed before the rest of `cid.h` has been processed, resulting in an 'implicit declaration of function' compiler error for `scx_cpu_arg()`. This violates the kernel coding standard requiring headers to be self-contained and independently compilable.
--

--- Patch [2]: [PATCH 2/4] sched_ext: Move scx_dispatch_sched() from sub.h to internal.h ---
commit 6f0aa181637de48d619823a3e1db41e281070615
Author: Tejun Heo <tj@kernel.org>

sched_ext: Move scx_dispatch_sched() from sub.h to internal.h

This commit moves scx_dispatch_sched() from sub.h to internal.h,
and includes cid.h at the end of internal.h to support scx_cpu_arg().

> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
> index d7a1d6a14ebf3..88b2a9b5d3ab2 100644
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -2289,4 +2289,115 @@ static inline struct scx_sched *scx_prog_sched(const struct bpf_prog_aux *aux)
>  static inline struct scx_sched *scx_parent(struct scx_sched *sch) { return NULL; }
>  #endif	/* CONFIG_EXT_SUB_SCHED */
>  
> +#include "cid.h"

[Severity: Low]
Does this create a circular header dependency between internal.h and cid.h?

If a C file includes cid.h before internal.h, cid.h includes internal.h
at the top. When processing internal.h, the #include "cid.h" here is
skipped by the include guard.

This causes scx_dispatch_sched() to be parsed before the rest of cid.h is
processed, which would result in an implicit declaration error for
scx_cpu_arg() further down in this function.

Could this cause build failures if headers are included in a different order
in the future?

> +
> +/*
> + * One user of this function is scx_bpf_dispatch() which can be called
> + * recursively as sub-sched dispatches nest. Always inline to reduce stack usage
> + * from the call frame.
> + */
> +static __always_inline bool
> +scx_dispatch_sched(struct scx_sched *sch, struct rq *rq,

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260714230917.84158-1-tj@kernel.org?part=2

Re: [PATCHSET sched_ext/for-7.3] sched_ext: Sub-scheduler follow-ups

[Andrea Righi]

Hi Tejun,

On Tue, Jul 14, 2026 at 01:09:13PM -1000, Tejun Heo wrote:
> Hello,
> 
> Follow-ups to the sub-scheduler capability series, addressing two things
> Andrea raised while reviewing v5[1]: sched teardown drained queued ecaps
> syncs with an msleep() poll loop, and with CONFIG_EXT_SUB_SCHED=y hot paths
> keep paying for sub-sched bookkeeping even when no sub-scheduler exists.
> 
> Patch 0001 replaces the teardown poll loop with direct removal from the
> llist. Patches 0002-0003 are prep and 0004 adds the scx_has_subs static key
> and gates the sub-sched portions of hot paths on it.

Thanks for addressing the issues! For the whole series:

Reviewed-by: Andrea Righi <arighi@nvidia.com>

-Andrea

> 
> [1] https://lore.kernel.org/r/20260709225041.1695495-1-tj@kernel.org
> 
> Based on sched_ext/for-7.3 (35f9cbbacb67).
> 
> This patchset contains the following 4 patches.
> 
>  0001 sched_ext: Remove queued ecaps syncs directly on sched teardown
>  0002 sched_ext: Move scx_dispatch_sched() from sub.h to internal.h
>  0003 sched_ext: Gate sub_dispatch_prev with CONFIG_EXT_SUB_SCHED
>  0004 sched_ext: Add the scx_has_subs static key and gate sub-sched hot paths
> 
> The patchset is also available in the following git branch:
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-sub-followups
> 
> diffstat follows. Thanks.
> 
>  kernel/sched/ext/ext.c      |   6 +-
>  kernel/sched/ext/idle.c     |  10 ++++
>  kernel/sched/ext/internal.h | 133 +++++++++++++++++++++++++++++++++++++++++++-
>  kernel/sched/ext/sub.c      |  88 ++++++++++++++++++++---------
>  kernel/sched/ext/sub.h      | 124 +++++------------------------------------
>  kernel/sched/sched.h        |   3 +-
>  6 files changed, 223 insertions(+), 141 deletions(-)
> 
> --
> tejun