linux-kernel - Re: Re: [PATCH 2/2] sched/fair: Fix premature check of WAKEUP

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Message-ID: <CAKfTPtBL4aPbEDOa8jJ+h2wQ9CLU80=0mQdrc07vfqBVswzAsg@mail.gmail.com>
Date: Mon, 24 Feb 2025 14:47:13 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Abel Wu <wuyun.abel@...edance.com>
Cc: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Josh Don <joshdon@...gle.com>, 
	Tianchen Ding <dtcccc@...ux.alibaba.com>, 
	"open list:SCHEDULER" <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

On Sun, 23 Feb 2025 at 12:22, Abel Wu <wuyun.abel@...edance.com> wrote:
>
> On 2/23/25 6:25 PM, Madadi Vineeth Reddy Wrote:
> > On 23/02/25 14:14, Abel Wu wrote:
> >> Hi Madadi,
> >>
> >> On 2/23/25 2:16 AM, Madadi Vineeth Reddy Wrote:
> >>> On 21/02/25 21:27, Abel Wu wrote:
> >>>> On 2/21/25 7:49 PM, Vincent Guittot Wrote:
> >>>>> On Fri, 21 Feb 2025 at 12:12, Abel Wu <wuyun.abel@...edance.com> wrote:
> >>>>>>
> >>>>>> Idle tasks are by definition preempted by non-idle tasks whether feat
> >>>>>> WAKEUP_PREEMPTION is enabled or not. This isn't true any longer since
> >>>>>
> >>>>> I don't think it's true, only "sched_idle never preempts others" is
> >>>>> always true but sched_feat(WAKEUP_PREEMPTION) is mainly there for
> >>>>> debug purpose so if WAKEUP_PREEMPTION is false then nobody preempts
> >>>>> others at wakeup, idle, batch or normal
> >>>>
> >>>> Hi Vincent, thanks for your comment!
> >>>>
> >>>> The SCHED_IDLE "definition" of being preempted by non-idle tasks comes
> >>>> from commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation")
> >>>> which said:
> >>>>
> >>>>       - no SCHED_IDLE buddies
> >>>>       - never let SCHED_IDLE preempt on wakeup
> >>>>       - always preempt SCHED_IDLE on wakeup
> >>>>       - limit SLEEPER fairness for SCHED_IDLE
> >>>>
> >>>> and that commit let it be preempted before checking WAKEUP_PREEMPTION.
> >>>> The rules were introduced in 2009, and to the best of my knowledge there
> >>>> seemed no behavior change ever since. Please correct me if I missed
> >>>> anything.
> >>>
> >>> As Vincent mentioned, WAKEUP_PREEMPTION is primarily for debugging. Maybe
> >>> it would help to document that SCHED_IDLE tasks are not preempted by non-idle
> >>> tasks when WAKEUP_PREEMPTION is disabled. Otherwise, the intent of having no
> >>> preemptions for debugging would be lost.
> >>>
> >>> Thoughts?
> >>
> >> I am not sure I really understand the purpose of this debug feature.
> >> If it wants to provide a way to check whether a performance degrade of
> >> certain workload is due to overscheduling or not, then do we really
> >> care about performance of SCHED_IDLE workloads and why?
> >
> > It's true that we may not be too concerned about performance with
> > SCHED_IDLE. The issue is preserve the original SCHED_IDLE definition
> > versus WAKEUP_PREEMPTION, which applies across all policies. Since by
>
> Yes, exactly.
>
> > default the feature is true. I am not sure. Either way seems ok to me.
>
> Hi Vincent,
>
> Since Peter gave the priority to SCHED_IDLE semantics over WAKEUP_PREEMPTION
> in his commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation"),
> and the choice is kept unchanged for quite a long time until the recent merged
> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> which seemed not intend to change it, shall we restore the choice for now and
> leave the discussion of the scope of WAKEUP_PREEMPTION to the future once any
> usecase shows up?

Or we should just remove it. I'm curious to know who used it during
the last couple of years ? Having in mind that lazy preemption adds
another level as check_preempt_wakeup_fair()  uses it so sched-idle
tasks might not always be immediately preempted anyway.


>
> Thanks,
>         Abel
>