linux-kernel - [PATCH v3 3/6] sched/fair: Remove spurious shorter slice preemption

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Message-ID: <20250708165630.1948751-4-vincent.guittot@linaro.org>
Date: Tue,  8 Jul 2025 18:56:27 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	vschneid@...hat.com,
	dhaval@...nis.ca,
	linux-kernel@...r.kernel.org
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v3 3/6] sched/fair: Remove spurious shorter slice preemption

Even if the waking task can preempt current, it might not be the one
selected by pick_task_fair. Check that the waking task will be selected
if we cancel the slice protection before doing so.

Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 44 ++++++++++++++------------------------------
 1 file changed, 14 insertions(+), 30 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 97cf99bb71d6..7e82b357763a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -931,7 +931,7 @@ 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)
+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);
@@ -948,7 +948,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
 	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
 		curr = NULL;
 
-	if (curr && protect_slice(curr))
+	if (curr && protect && protect_slice(curr))
 		return curr;
 
 	/* Pick the leftmost entity if it's eligible */
@@ -992,6 +992,11 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
 	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);
@@ -1175,27 +1180,6 @@ static inline bool resched_next_quantum(struct cfs_rq *cfs_rq, struct sched_enti
 	return !entity_eligible(cfs_rq, curr);
 }
 
-static inline bool do_preempt_short(struct cfs_rq *cfs_rq,
-				    struct sched_entity *pse, struct sched_entity *se)
-{
-	if (!sched_feat(PREEMPT_SHORT))
-		return false;
-
-	if (pse->slice >= se->slice)
-		return false;
-
-	if (!entity_eligible(cfs_rq, pse))
-		return false;
-
-	if (entity_before(pse, se))
-		return true;
-
-	if (!entity_eligible(cfs_rq, se))
-		return true;
-
-	return false;
-}
-
 /*
  * Used by other classes to account runtime.
  */
@@ -8666,6 +8650,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	struct sched_entity *se = &donor->se, *pse = &p->se;
 	struct cfs_rq *cfs_rq = task_cfs_rq(donor);
 	int cse_is_idle, pse_is_idle;
+	bool do_preempt_short = false;
 
 	if (unlikely(se == pse))
 		return;
@@ -8714,7 +8699,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 		 * When non-idle entity preempt an idle entity,
 		 * don't give idle entity slice protection.
 		 */
-		cancel_protect_slice(se);
+		do_preempt_short = true;
 		goto preempt;
 	}
 
@@ -8732,22 +8717,21 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	/*
 	 * If @p has a shorter slice than current and @p is eligible, override
 	 * current's slice protection in order to allow preemption.
-	 *
-	 * Note that even if @p does not turn out to be the most eligible
-	 * task at this moment, current's slice protection will be lost.
 	 */
-	if (do_preempt_short(cfs_rq, pse, se))
-		cancel_protect_slice(se);
+	do_preempt_short = sched_feat(PREEMPT_SHORT) && (pse->slice < se->slice);
 
 	/*
 	 * If @p has become the most eligible task, force preemption.
 	 */
-	if (pick_eevdf(cfs_rq) == pse)
+	if (__pick_eevdf(cfs_rq, !do_preempt_short) == pse)
 		goto preempt;
 
 	return;
 
 preempt:
+	if (do_preempt_short)
+		cancel_protect_slice(se);
+
 	resched_curr_lazy(rq);
 }
 
-- 
2.43.0