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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKfTPtC1hK7Cpy-+0EmSzB+zsWOZFZau2+LAckHY9CfJV4tvCw@mail.gmail.com>
Date: Tue, 25 Feb 2025 18:14:17 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Phil Auld <pauld@...hat.com>
Cc: Abel Wu <wuyun.abel@...edance.com>, 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 Mon, 24 Feb 2025 at 15:10, Phil Auld <pauld@...hat.com> wrote:
>
> On Mon, Feb 24, 2025 at 02:47:13PM +0100 Vincent Guittot wrote:
> > 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.
> >
>
> It can be helpful to be able to turn that off when chasing performance
> issues. See the DELAY_DEQUEUE thread from a few months back. In that
> case we never got to a good answer, but did use NO_WAKEUP_PREEMPTION
> during debugging to take out some variables at least. FWIW.

ok, I didn't remember it has been used

>
>
> Cheers,
> Phil
>
> >
> > >
> > > Thanks,
> > >         Abel
> > >
> >
>
> --
>