linux-kernel - Re: [PATCH v2] sched/fair: Fix detection of per-CPU kthreads waking a task

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Date:   Fri, 3 Dec 2021 09:34:08 +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 v2] sched/fair: Fix detection of per-CPU kthreads waking a task

On Wed, 1 Dec 2021 at 15:35, Vincent Donnefort
<vincent.donnefort@....com> wrote:
>
> select_idle_sibling() has a special case for tasks woken up by a per-CPU
> kthread, where the selected CPU is the previous one. However, the current
> condition for this exit path is incomplete. A task can wake up from an
> interrupt context (e.g. hrtimer), while a per-CPU kthread is running. A
> such scenario would spuriously trigger the special case described above.
> Also, a recent change made the idle task like a regular per-CPU kthread,
> hence making that situation more likely to happen
> (is_per_cpu_kthread(swapper) being true now).
>
> Checking for task context makes sure select_idle_sibling() will not
> interpret a wake up from any other context as a wake up by a per-CPU
> kthread.
>
> 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>

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

>
> ---
> v1 -> v2:
>   * is_idle_thread() -> in_task() to also include spurious detection when
>     current != swapper. (Vincent Guittot)
> ---
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 945d987246c5..56db4ae85995 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) &&
> +           in_task() &&
>             prev == smp_processor_id() &&
>             this_rq()->nr_running <= 1) {
>                 return prev;
> --
> 2.25.1
>