linux-kernel - Re: [PATCH] sched/fair: Make SCHED_IDLE se be preempted in strict hierarchy

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Date: Tue, 25 Jun 2024 15:36:55 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Tianchen Ding <dtcccc@...ux.alibaba.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, Josh Don <joshdon@...gle.com>
Subject: Re: [PATCH] sched/fair: Make SCHED_IDLE se be preempted in strict hierarchy

On Mon, 24 Jun 2024 at 09:39, Tianchen Ding <dtcccc@...ux.alibaba.com> wrote:
>
> Consider the following cgroup:
>                        root
>                         |
>              ------------------------
>              |                      |
>        normal_cgroup            idle_cgroup
>              |                      |
>    SCHED_IDLE task_A           SCHED_NORMAL task_B
>
> According to the cgroup hierarchy, A should preempt B. But current
> check_preempt_wakeup_fair() treats cgroup se and task separately, so B
> will preempt A unexpectedly.
> Unify the wakeup logic by {p}se_is_idle only.
>
> Also fix a bug about se_is_idle() definition when
> !CONFIG_FAIR_GROUP_SCHED.
>
> Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
> Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
> ---
>  kernel/sched/fair.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 41b58387023d..c91cfaa7d9ee 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -511,7 +511,7 @@ static int cfs_rq_is_idle(struct cfs_rq *cfs_rq)
>
>  static int se_is_idle(struct sched_entity *se)
>  {
> -       return 0;
> +       return task_has_idle_policy(task_of(se));
>  }
>
>  #endif /* CONFIG_FAIR_GROUP_SCHED */
> @@ -8382,16 +8382,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>         if (test_tsk_need_resched(curr))
>                 return;
>
> -       /* Idle tasks are by definition preempted by non-idle tasks. */
> -       if (unlikely(task_has_idle_policy(curr)) &&
> -           likely(!task_has_idle_policy(p)))
> -               goto preempt;
> -
> -       /*
> -        * Batch and idle tasks do not preempt non-idle tasks (their preemption
> -        * is driven by the tick):
> -        */
> -       if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
> +       if (!sched_feat(WAKEUP_PREEMPTION))
>                 return;
>
>         find_matching_se(&se, &pse);
> @@ -8408,6 +8399,12 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int

Replace "group" by "entity" in the comment above as it is not only
group but also entity

>                 goto preempt;
>         if (cse_is_idle != pse_is_idle)
>                 return;
> +       /*
> +        * Batch tasks do not preempt non-idle tasks (their preemption
> +        * is driven by the tick):
> +        */
> +       if (unlikely(pse == &p->se && p->policy == SCHED_BATCH))

I think I would prefer entity_is_task() which makes easier to
understand that the condition is about task

+       if (unlikely(entity_is_task(pse) &&  p->policy == SCHED_BATCH))

other than the 2 comments above

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>


> +               return;
>
>         cfs_rq = cfs_rq_of(se);
>         update_curr(cfs_rq);
> --
> 2.39.3
>