linux-kernel - Re: Re: [PATCH] sched/fair: remove next_buddy

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Date:   Thu, 14 Dec 2023 14:41:07 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Wang Jinchao <wangjinchao@...sion.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, stone.xulei@...sion.com
Subject: Re: Re: [PATCH] sched/fair: remove next_buddy_marked

On Thu, 14 Dec 2023 at 13:23, Abel Wu <wuyun.abel@...edance.com> wrote:
>
> On 12/14/23 4:18 PM, Vincent Guittot Wrote:
> > On Thu, 14 Dec 2023 at 06:20, Wang Jinchao <wangjinchao@...sion.com> wrote:
> >>
> >> Remove unused `next_buddy_marked` in `check_preempt_wakeup_fair`
> >>
> >
> > Fixes: 5e963f2bd465 ("sched/fair: Commit to EEVDF")
>
> After this commit @pse preempts curr without being the NEXT_BUDDY, but
> IMHO it should be, so how about this?
>
> @@ -8259,8 +8259,11 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>          /*
>           * XXX pick_eevdf(cfs_rq) != se ?
>           */
> -       if (pick_eevdf(cfs_rq) == pse)
> +       if (pick_eevdf(cfs_rq) == pse) {
> +               if (!next_buddy_marked)
> +                       set_next_buddy(pse);

I don't think this is needed because :
- NEXT_BUDDY is false by default so pick_next_entity() will not take
care of this
- pick_next_entity() will call pick_eevdf() which should return pse
unless another se that want to run 1st, wakes up in the meantime and
we should probably not take into account next buddy in this case

>                  goto preempt;
> +       }
>
>          return;
>
> which will align with before.