Re: [PATCH] sched/fair: Disable scheduler feature NEXT_BUDDY

[Peter Zijlstra <peterz@infradead.org>]

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?