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

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Message-ID: <9d9d7432-9a5d-4216-ac53-a0f333a35d8f@bytedance.com>
Date: Sun, 23 Feb 2025 16:44:52 +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

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?

IMHO preempting SCHED_IDLE before WAKEUP_PREEMPTION is to preserve the
IDLE semantics trying to behave like real idle task. It is somehow
weird to me that we treat sched-idle cpus as idle while don't let the
non-idle tasks run immediately on sched-idle cpus on debug case.

Thanks,
	Abel