Commit 9de74a98 authored by Vincent Guittot's avatar Vincent Guittot Committed by Peter Zijlstra
Browse files

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: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-4-vincent.guittot@linaro.org
parent 74eec636

kernel/sched/fair.c

+14 −30
Original line number Diff line number Diff line
@@ -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);

}