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

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Date:   Thu, 14 Dec 2023 21:02:17 +0800
From:   Wang Jinchao <wangjinchao@...sion.com>
To:     Abel Wu <wuyun.abel@...edance.com>
CC:     Vincent Guittot <vincent.guittot@...aro.org>,
        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, Dec 14, 2023 at 08:21:53PM +0800, Abel Wu 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);
>                 goto preempt;
> +       }
> 
>         return;
> 
> which will align with before.
Seizing this opportunity to inquire about a question:
What does "buddy" mean in the context of the scheduler?

Is the effect the same between 
    preempting after pick_evfd(cfs_rq) == pse 
and 
    preempting after set_next_buddy(pse) followed by pick_evfd(cfs_rq) == pse?
Would both scenarios result in pse becoming the next scheduled se?"