Re: [PATCH 1/2] sched/fair: Do not let idle entities preempt others

[Vincent Guittot]

On Fri, 21 Feb 2025 at 12:12, Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> A task with SCHED_IDLE policy doesn't preempt others by definition, and
> the semantics are intended to be preserved when extending to cgroups
> introduced in commit 304000390f88 ("sched: Cgroup SCHED_IDLE support").
>
> But current implementation allows idle entities to preempt each other
> on wakeup, which seems not behave as expected especially after
> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> so fix this by explicitly skip wakeup preemption for idle entities.
>
> Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1c0ef435a7aa..4340178f29b7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8778,12 +8778,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>         pse_is_idle = se_is_idle(pse);
>
>         /*
> -        * Preempt an idle entity in favor of a non-idle entity (and don't preempt
> -        * in the inverse case).
> +        * Preempt an idle entity in favor of a non-idle entity.
>          */
>         if (cse_is_idle && !pse_is_idle)
>                 goto preempt;
> -       if (cse_is_idle != pse_is_idle)
> +
> +       /*
> +        * IDLE entities do not preempt others.
> +        */
> +       if (unlikely(pse_is_idle))
>                 return;

Fair enough

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

>
>         /*

> --
> 2.37.3
>