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

[Abel Wu]

On 12/14/23 9:02 PM, Wang Jinchao Wrote:
> 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?

struct sched_entity

> 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?"

Probably, since pse is the one preempts curr, pick_next_entity() could
return pse directly without walking the rbtree. So the difference is in
performance.