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

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Message-ID: <83f8b833-af79-4d77-a179-5065aec994dc@bytedance.com>
Date: Sun, 23 Feb 2025 19:22:33 +0800
From: Abel Wu <wuyun.abel@...edance.com>
To: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
 Vincent Guittot <vincent.guittot@...aro.org>
Cc: 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 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?

Thanks,
	Abel