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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 19 Jan 2021 14:34:55 +0100
Message-ID: <CAKfTPtD+wU9-UitvnBmZFeAgXbsDO+qcsv-hYr0xxuewB_Kp+A@mail.gmail.com>
Subject: Re: [PATCH] sched/eas: Don't update misfit status if the task is pinned
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
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On Tue, 19 Jan 2021 at 13:08, Qais Yousef <qais.yousef@arm.com> wrote:
>
> If the task is pinned to a cpu, setting the misfit status means that
> we'll unnecessarily continuously attempt to migrate the task but fail.
>
> This continuous failure will cause the balance_interval to increase to
> a high value, and eventually cause unnecessary significant delays in
> balancing the system when real imbalance happens.
>
> Caught while testing uclamp where rt-app calibration loop was pinned to
> cpu 0, shortly after which we spawn another task with high util_clamp
> value. The task was failing to migrate after over 40ms of runtime due to
> balance_interval unnecessary expanded to a very high value from the
> calibration loop.
>
> Not done here, but it could be useful to extend the check for pinning to
> verify that the affinity of the task has a cpu that fits. We could end
> up in a similar situation otherwise.
>
> Fixes: 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 197a51473e0c..9379a481dd8c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4060,7 +4060,7 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
>         if (!static_branch_unlikely(&sched_asym_cpucapacity))
>                 return;
>
> -       if (!p) {
> +       if (!p || p->nr_cpus_allowed == 1) {

Side question: What happens if there is 2 misfit tasks and the current
one is pinned but not the other waiting one

>                 rq->misfit_task_load = 0;
>                 return;
>         }
> --
> 2.25.1
>