linux-kernel - Re: [PATCH 8/8] sched/fair: Relax task

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Date:   Mon, 8 Feb 2021 17:21:50 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 8/8] sched/fair: Relax task_hot() for misfit tasks

On Thu, 28 Jan 2021 at 19:32, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> Misfit tasks can and will be preempted by the stopper to migrate them over
> to a higher-capacity CPU. However, when runnable but not current misfit
> tasks are scanned by the load balancer (i.e. detach_tasks()), the
> task_hot() ratelimiting logic may prevent us from enqueuing said task onto
> a higher-capacity CPU.
>
> Align detach_tasks() with the active-balance logic and let it pick a
> cache-hot misfit task when the destination CPU can provide a capacity
> uplift.
>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cba9f97d9beb..c2351b87824f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7484,6 +7484,17 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
>         if (env->sd->flags & SD_SHARE_CPUCAPACITY)
>                 return 0;
>
> +       /*
> +        * On a (sane) asymmetric CPU capacity system, the increase in compute
> +        * capacity should offset any potential performance hit caused by a
> +        * migration.
> +        */
> +       if (sd_has_asym_cpucapacity(env->sd) &&
> +           env->idle != CPU_NOT_IDLE &&
> +           !task_fits_capacity(p, capacity_of(env->src_cpu)) &&
> +           cpu_capacity_greater(env->dst_cpu, env->src_cpu))

Why not using env->migration_type to directly detect that it's a
misfit task active migration ?

> +               return 0;
> +
>         /*
>          * Buddy candidates are cache hot:
>          */
> --
> 2.27.0
>