Re: Re: [PATCH] sched/fair: remove next_buddy_marked

[Wang Jinchao]

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@xxxxxxxxxxx> 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?"