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@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-4-vincent.guittot@linaro.org

Author: vingu-linaro

kernel/sched/fair.c

@@ -932,7 +932,7 @@ 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);
@@ -949,7 +949,7 @@ 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 */
@@ -993,6 +993,11 @@ 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);
@@ -1176,27 +1181,6 @@ static inline bool resched_next_slice(struct cfs_rq *cfs_rq, struct sched_entity
 	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.
  */
@@ -8658,6 +8642,7 @@ 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;
@@ -8706,7 +8691,7 @@ 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;
 	}
 
@@ -8724,22 +8709,21 @@ 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);
 }