linux-kernel - Re: [PATCH] sched/fair: tweak pick_next

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Date:   Thu, 1 Oct 2020 09:17:43 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Oskolkov <posk@...k.io>
Subject: Re: [PATCH] sched/fair: tweak pick_next_entity

On Wed, 30 Sep 2020 at 19:35, Peter Oskolkov <posk@...gle.com> wrote:
>
> Currently, pick_next_entity(...) has the following structure
> (simplified):
>
> [...]
> if (last_buddy_ok())
>   result = last_buddy;
> if (next_buddy_ok())
>   result = next_buddy;
> [...]
>
> The intended behavior is to prefer next buddy over last buddy;
> the current code somewhat obfuscates this, and also wastes
> cycles checking the last buddy when eventually the next buddy is
> picked up.
>
> So this patch refactors two 'ifs' above into
>
> [...]
> if (next_buddy_ok())
>     result = next_buddy;
> else if (last_buddy_ok())
>     result = last_buddy;
> [...]
>
> Signed-off-by: Peter Oskolkov <posk@...gle.com>

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

> ---
>  kernel/sched/fair.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fc3410b8b990..cec6cf9b2bb3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>                         se = second;
>         }
>
> -       /*
> -        * Prefer last buddy, try to return the CPU to a preempted task.
> -        */
> -       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> -               se = cfs_rq->last;
> -
> -       /*
> -        * Someone really wants this to run. If it's not unfair, run it.
> -        */
> -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> +       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> +               /*
> +                * Someone really wants this to run. If it's not unfair, run it.
> +                */
>                 se = cfs_rq->next;
> +       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
> +               /*
> +                * Prefer last buddy, try to return the CPU to a preempted task.
> +                */
> +               se = cfs_rq->last;
> +       }
>
>         clear_buddies(cfs_rq, se);
>
> --
> 2.28.0.709.gb0816b6eb0-goog
>