linux-kernel - Re: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

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Date:   Wed, 22 Dec 2021 09:20:17 +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, dietmar.eggemann@....com,
        Valentin.Schneider@....com, Morten.Rasmussen@....com,
        qperret@...gle.com
Subject: Re: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

On Mon, 20 Dec 2021 at 12:43, Vincent Donnefort
<vincent.donnefort@....com> wrote:
>
> During a migration, the lock for the previous runqueue is not taken and
> hence, the task contribution isn't directly removed from that runqueue
> utilization but instead temporarily saved, until the next PELT signals
> update where it would be accounted. There is then a window in which a
> CPU can ben idle be nonetheless overutilized.
>
> The load balancer wouldn't be able to do anything to help a sleeping CPU,
> it brings then no gain to raise overutilized there, only the risk of
> spuriously doing it.

But how do you make the difference between a very short idle time of
an overutilized CPU and a idle cpu with outdated utilization

Being idle is not a good reason for not being overutilized (ie ~80% of
average utilisation)

>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 51f6f55abb37..37f737c5f0b8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8641,26 +8641,28 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>
>                 nr_running = rq->nr_running;
>                 sgs->sum_nr_running += nr_running;
> -
> -               if (nr_running > 1)
> -                       *sg_status |= SG_OVERLOAD;
> -
> -               if (cpu_overutilized(i))
> -                       *sg_status |= SG_OVERUTILIZED;
> -
>  #ifdef CONFIG_NUMA_BALANCING
>                 sgs->nr_numa_running += rq->nr_numa_running;
>                 sgs->nr_preferred_running += rq->nr_preferred_running;
>  #endif
> +               if (nr_running > 1)
> +                       *sg_status |= SG_OVERLOAD;

Why do you move this code related to overload ?

> +
>                 /*
>                  * No need to call idle_cpu() if nr_running is not 0
>                  */
>                 if (!nr_running && idle_cpu(i)) {
>                         sgs->idle_cpus++;
> -                       /* Idle cpu can't have misfit task */
> +                       /*
> +                        * Idle cpu can neither be overutilized nor have a
> +                        * misfit task.
> +                        */
>                         continue;
>                 }
>
> +               if (cpu_overutilized(i))
> +                       *sg_status |= SG_OVERUTILIZED;
> +
>                 if (local_group)
>                         continue;
>
> --
> 2.25.1
>