linux-kernel - Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task

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Message-ID: <CAKfTPtDX8sOfguZhJt5QV3j5D_JetcgncuF2w+uLa0XDk7UXkw@mail.gmail.com>
Date:   Thu, 25 Nov 2021 10:05:58 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Vincent Donnefort <Vincent.Donnefort@....com>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        dietmar.eggemann@....com, Valentin.Schneider@....com
Subject: Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task

On Wed, 24 Nov 2021 at 16:42, Vincent Donnefort
<vincent.donnefort@....com> wrote:
>
> select_idle_sibling() will return prev_cpu for the case where the task is
> woken up by a per-CPU kthread. However, the idle task has been recently
> modified and is now identified by is_per_cpu_kthread(), breaking the
> behaviour described above. Using !is_idle_task() ensures we do not
> spuriously trigger that select_idle_sibling() exit path.
>
> Fixes: 00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 945d987246c5..8bf95b0e368d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6399,6 +6399,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>          * pattern is IO completions.
>          */
>         if (is_per_cpu_kthread(current) &&
> +           !is_idle_task(current) &&
>             prev == smp_processor_id() &&
>             this_rq()->nr_running <= 1) {
>                 return prev;

AFAICT, this can't be possible for a symmetric system because it would
have been already returned by other conditions.
Only an asymmetric system can face such a situation if the task
doesn't fit which is the subject of your other patch.
so this patch seems irrelevant outside the other one


> --
> 2.25.1
>