linux-kernel - Re: [PATCH] sched/fair: Disable scheduler feature NEXT

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Message-ID: <20260123100915.GL171111@noisy.programming.kicks-ass.net>
Date: Fri, 23 Jan 2026 11:09:15 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ryan Roberts <ryan.roberts@....com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Ingo Molnar <mingo@...hat.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Chris Mason <clm@...a.com>, linux-kernel@...r.kernel.org,
	Christian.Loehle@....com
Subject: Re: [PATCH] sched/fair: Disable scheduler feature NEXT_BUDDY

On Fri, Jan 23, 2026 at 11:04:20AM +0100, Vincent Guittot wrote:
> On Fri, 23 Jan 2026 at 10:53, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Jan 22, 2026 at 06:34:28PM +0100, Vincent Guittot wrote:
> >
> > > The new NEXT_BUDDY implementation is doing more than setting a buddy;
> > > it also breaks the run to parity mechanism by always setting next
> > > buddy during wakeup_preempt_fair() even if there is no relation
> > > between the 2 tasks and PICK_BUDDY bypasses protections
> > >
> > > In addition to disable NEXT_BUDDY, i suggest to also revert the force
> > > preemption section below which also breaks run_to_parity by doing an
> > > assumption whereas WF_SYNC is normally there for such purpose
> > >
> > > -- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -8822,16 +8822,6 @@ static void wakeup_preempt_fair(struct rq *rq,
> > > struct task_struct *p, int wake_f
> > >         if ((wake_flags & WF_FORK) || pse->sched_delayed)
> > >                 return;
> > >
> > > -       /*
> > > -        * If @p potentially is completing work required by current then
> > > -        * consider preemption.
> > > -        *
> > > -        * Reschedule if waker is no longer eligible. */
> > > -       if (in_task() && !entity_eligible(cfs_rq, se)) {
> > > -               preempt_action = PREEMPT_WAKEUP_RESCHED;
> > > -               goto preempt;
> > > -       }
> > > -
> > >         /* Prefer picking wakee soon if appropriate. */
> > >         if (sched_feat(NEXT_BUDDY) &&
> > >             set_preempt_buddy(cfs_rq, wake_flags, pse, se)) {
> > >
> > > This largely increases the number of resched and preemption because a
> > > task becomes quickly "ineligible": We update our internal vruntime
> > > periodically and before the task exhausted its slice.
> >
> > Hmm, fair enough. Do I munge that into Mel's patch, or should I create a
> > second patch from you for this?
> 
> I can prepare a patch with description and sent it right now if you want

Sure that works. Then I'll stick both into tip/sched/urgent or
thereabout :-)