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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 15 Jun 2020 17:09:51 +0200
Message-ID: <CAKfTPtACgUutP86nM6HvgQ-Hj7RdBnK+yxqg3Dr+cOZCPSoeyg@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Optimize dequeue_task_fair()
To:     Peng Wang <rocking@linux.alibaba.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Mon, 15 Jun 2020 at 16:20, Peng Wang <rocking@linux.alibaba.com> wrote:
>
> While looking at enqueue_task_fair and dequeue_task_fair, it occurred
> to me that dequeue_task_fair can also be optimized as Vincent described
> in commit 7d148be69e3a ("sched/fair: Optimize enqueue_task_fair()").

good point

>
> dequeue_throttle label can ensure se not to be NULL, and se is
> always NULL when reaching root level.
>
> Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cbcb2f7..e6be952 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5614,10 +5614,10 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>
>         }
>
> -dequeue_throttle:
> -       if (!se)
> -               sub_nr_running(rq, 1);
> +       /* At this point se is NULL and we are at root level*/
> +       sub_nr_running(rq, 1);
>
> +dequeue_throttle:
>         /* balance early to pull high priority tasks */
>         if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
>                 rq->next_balance = jiffies;

You can move the label here because sched_idle_rq() uses
rq->nr-running and rq->cfs.idle_h_nr_running so they will not change
if we jump to the label before reaching root level

> --
> 2.9.5
>