Re: [PATCH v2 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

[Vincent Guittot]

On Thu, 27 Feb 2025 at 09:58, Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> The commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation")
> defines the behavior of SCHED_IDLE as following:
>
>  - no SCHED_IDLE buddies
>  - never let SCHED_IDLE preempt on wakeup
>  - always preempt SCHED_IDLE on wakeup
>  - limit SLEEPER fairness for SCHED_IDLE
>
> and the 3rd rule is broken if !WAKEUP_PREEMPTION due to recently merged
> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
>
> Although WAKEUP_PREEMPTION is mainly there for debug purpose to provide
> a way to check whether a performance degrade of certain workload is due
> to overscheduling or not, it is still kind of weird that we treat sched-
> idle cpus as idle but don't let the non-idle tasks preempt the sched-idle
> cpus in debug mode (!WAKEUP_PREEMPTION).
>
> Fix it by strictly following the aforementioned rules.
>
> Fixes: faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>

WAKEUP_PREEMPTION seems to be still used so

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

> ---
>  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 aaa9822e9562..a7eeb72d57aa 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8785,9 +8785,6 @@ 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);
>
> @@ -8806,6 +8803,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>                 goto preempt;
>         }
>
> +       if (!sched_feat(WAKEUP_PREEMPTION))
> +               return;
> +
>         /*
>          * IDLE entities do not preempt others.
>          */
> --
> 2.37.3
>