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

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/01/2026 10:42, Vincent Guittot wrote:
> On Fri, 23 Jan 2026 at 11:09, Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> On Fri, Jan 23, 2026 at 11:04:20AM +0100, Vincent Guittot wrote:
>>> On Fri, 23 Jan 2026 at 10:53, Peter Zijlstra <peterz@infradead.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 :-)
> 
> I sent it.
> https://lore.kernel.org/all/20260123102858.52428-1-vincent.guittot@linaro.org/

This is needed in addition to Mel's patch to disable NEXT_BUDDY, right? I'll
kick off another benchmark run and report back on Monday.