Re: [PATCH 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

[Vincent Guittot]

On Fri, 21 Feb 2025 at 12:12, Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> Idle tasks are by definition preempted by non-idle tasks whether feat
> WAKEUP_PREEMPTION is enabled or not. This isn't true any longer since

I don't think it's true, only "sched_idle never preempts others" is
always true but sched_feat(WAKEUP_PREEMPTION) is mainly there for
debug purpose so if WAKEUP_PREEMPTION is false then nobody preempts
others at wakeup, idle, batch or normal


> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> which gives priority to WAKEUP_PREEMPTION, so when !FAIR_GROUP_SCHED,
> SCHED_IDLE tasks do not preempt by non-idle tasks.
>
> Fixes: faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4340178f29b7..de9a2689de9c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8768,9 +8768,6 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>         if (test_tsk_need_resched(rq->curr))
>                 return;
>
> -       if (!sched_feat(WAKEUP_PREEMPTION))
> -               return;
> -
>         find_matching_se(&se, &pse);
>         WARN_ON_ONCE(!pse);
>
> @@ -8783,6 +8780,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>         if (cse_is_idle && !pse_is_idle)
>                 goto preempt;
>
> +       if (!sched_feat(WAKEUP_PREEMPTION))
> +               return;
> +
>         /*
>          * IDLE entities do not preempt others.
>          */
> --
> 2.37.3
>