linux-kernel - Re: [PATCH] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

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Date:   Thu, 25 Nov 2021 10:10:01 +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 per-CPU kthread and wakee stacking for
 asym CPU capacity

On Wed, 24 Nov 2021 at 15:15, Vincent Donnefort
<vincent.donnefort@....com> wrote:
>
> A shortcut has been introduced in select_idle_sibling() to return prev_cpu
> if the wakee is woken up by a per-CPU kthread. This is an issue for
> asymmetric CPU capacity systems where the wakee might not fit prev_cpu
> anymore. Evaluate asym_fits_capacity() for prev_cpu before using that
> shortcut.
>
> Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6291876a9d32..b90dc6fd86ca 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6410,7 +6410,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>          */
>         if (is_per_cpu_kthread(current) &&
>             prev == smp_processor_id() &&
> -           this_rq()->nr_running <= 1) {
> +           this_rq()->nr_running <= 1 &&
> +           asym_fits_capacity(task_util, prev)) {

With this patch, i don't think that
20211124154239.3191366-1-vincent.donnefort@....com is needed anymore

>                 return prev;
>         }
>
> --
> 2.25.1
>