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

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Message-ID: <e1cfabab-1326-4cd8-a8a4-4b3fc4c1f7ec@linux.ibm.com>
Date: Sat, 22 Feb 2025 23:46:48 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Abel Wu <wuyun.abel@...edance.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        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>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

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?

Thanks,
Madadi Vineeth Reddy

> 
> Best Regards,
>     Abel
>