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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 1 Jun 2021 16:11:28 +0200
Message-ID: <CAKfTPtDTxOA48R80cBqs+PXUc5jXg1VsYmyYt8Y=zy4N-wztFg@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Return early from update_tg_cfs_load() if
 delta == 0
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, 1 Jun 2021 at 10:36, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> In case the _avg delta is 0 there is no need to update se's _avg
> (level n) nor cfs_rq's _avg (level n-1). These values stay the same.
>
> Since cfs_rq's _avg isn't changed, i.e. no load is propagated down,
> cfs_rq's _sum should stay the same as well.
>
> So bail out after se's _sum has been updated.
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>
> This patch is against current tip/sched/urgent, commit f268c3737eca
> ("tick/nohz: Only check for RCU deferred wakeup on user/guest entry
> when needed").
> It needs commit 7c7ad626d9a0 ("sched/fair: Keep load_avg and load_sum
> synced").
>
>  kernel/sched/fair.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e7c8277e3d54..ce8e0e10e5d4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3548,9 +3548,12 @@ update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq
>         load_sum = (s64)se_weight(se) * runnable_sum;
>         load_avg = div_s64(load_sum, divider);
>
> +       se->avg.load_sum = runnable_sum;
> +
>         delta = load_avg - se->avg.load_avg;
> +       if (!delta)
> +               return;
>
> -       se->avg.load_sum = runnable_sum;
>         se->avg.load_avg = load_avg;
>
>         add_positive(&cfs_rq->avg.load_avg, delta);
> --
> 2.25.1
>