MIME-Version: 1.0
References: <20211124141435.3125147-1-vincent.donnefort@arm.com>
In-Reply-To: <20211124141435.3125147-1-vincent.donnefort@arm.com>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 25 Nov 2021 10:10:01 +0100
Message-ID: <CAKfTPtCrFoH-Kh0FAX_mWyjUymeHjCEOmJutL9qBpMftRe4Bdg@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Fix per-CPU kthread and wakee stacking for
 asym CPU capacity
To:     Vincent Donnefort <Vincent.Donnefort@arm.com>
Cc:     peterz@infradead.org, mingo@redhat.com,
        linux-kernel@vger.kernel.org, mgorman@techsingularity.net,
        dietmar.eggemann@arm.com, Valentin.Schneider@arm.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, 24 Nov 2021 at 15:15, Vincent Donnefort
<vincent.donnefort@arm.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@arm.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@arm.com is needed anymore

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