[tip: sched/urgent] sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue

[tip-bot2 for Vincent Guittot]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     ac8e69e693631689d74d8f1ebee6f84f737f797f
Gitweb:        https://git.kernel.org/tip/ac8e69e693631689d74d8f1ebee6f84f737f797f
Author:        Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate:    Wed, 22 Apr 2026 11:34:00 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 28 Apr 2026 09:19:54 +02:00

sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue

Similar to how pick_next_entity() must dequeue delayed entities, so too must
wakeup_preempt_fair(). Any delayed task being found means it is eligible and
hence past the 0-lag point, ready for removal.

Worse, by not removing delayed entities from consideration, it can skew the
preemption decision, with the end result that a short slice wakeup will not
result in a preemption.

                     tip/sched/core  tip/sched/core    +this patch
cyclictest slice  (ms) (default)2.8             8               8
hackbench slice   (ms) (default)2.8            20              20
Total Samples          |    22559           22595           22683
Average           (us) |      157              64( 59%)        59(  8%)
Median (P50)      (us) |       57              57(  0%)        58(- 2%)
90th Percentile   (us) |       64              60(  6%)        60(  0%)
99th Percentile   (us) |     2407              67( 97%)        67(  0%)
99.9th Percentile (us) |     3400            2288( 33%)       727( 68%)
Maximum           (us) |     5037            9252(-84%)      7461( 19%)

Fixes: f12e148892ed ("sched/fair: Prepare pick_next_task() for delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260422093400.319251-1-vincent.guittot@xxxxxxxxxx
---
 kernel/sched/fair.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 615861d..7289658 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1104,7 +1104,7 @@ static inline void cancel_protect_slice(struct sched_entity *se)
  *
  * Which allows tree pruning through eligibility.
  */
-static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq, bool protect)
+static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq, bool protect)
 {
 	struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
 	struct sched_entity *se = __pick_first_entity(cfs_rq);
@@ -1175,11 +1175,6 @@ found:
 	return best;
 }
 
-static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
-{
-	return __pick_eevdf(cfs_rq, true);
-}
-
 struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
 {
 	struct rb_node *last = rb_last(&cfs_rq->tasks_timeline.rb_root);
@@ -5754,11 +5749,11 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags);
  * 4) do not run the "skip" process, if something else is available
  */
 static struct sched_entity *
-pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
+pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq, bool protect)
 {
 	struct sched_entity *se;
 
-	se = pick_eevdf(cfs_rq);
+	se = pick_eevdf(cfs_rq, protect);
 	if (se->sched_delayed) {
 		dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
 		/*
@@ -9032,7 +9027,7 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 {
 	enum preempt_wakeup_action preempt_action = PREEMPT_WAKEUP_PICK;
 	struct task_struct *donor = rq->donor;
-	struct sched_entity *se = &donor->se, *pse = &p->se;
+	struct sched_entity *nse, *se = &donor->se, *pse = &p->se;
 	struct cfs_rq *cfs_rq = task_cfs_rq(donor);
 	int cse_is_idle, pse_is_idle;
 
@@ -9143,11 +9138,17 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 	}
 
 pick:
+	nse = pick_next_entity(rq, cfs_rq, preempt_action != PREEMPT_WAKEUP_SHORT);
+	/* If @p has become the most eligible task, force preemption */
+	if (nse == pse)
+		goto preempt;
+
 	/*
-	 * If @p has become the most eligible task, force preemption.
+	 * Because p is enqueued, nse being null can only mean that we
+	 * dequeued a delayed task.
 	 */
-	if (__pick_eevdf(cfs_rq, preempt_action != PREEMPT_WAKEUP_SHORT) == pse)
-		goto preempt;
+	if (!nse)
+		goto pick;
 
 	if (sched_feat(RUN_TO_PARITY))
 		update_protect_slice(cfs_rq, se);
@@ -9184,7 +9185,7 @@ again:
 
 		throttled |= check_cfs_rq_runtime(cfs_rq);
 
-		se = pick_next_entity(rq, cfs_rq);
+		se = pick_next_entity(rq, cfs_rq, true);
 		if (!se)
 			goto again;
 		cfs_rq = group_cfs_rq(se);