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sched/fair: Remove spurious shorter slice preemption

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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
This commit is contained in:
Vincent Guittot 2025-07-08 18:56:27 +02:00 committed by Peter Zijlstra
parent 74eec63661
commit 9de74a9850
1 changed files with 14 additions and 30 deletions
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44
kernel/sched/fair.c
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@ -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);
}