linux-kernel - Re: [PATCH 12/24] sched/fair: Prepare exit/cleanup paths for delayed

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Message-ID: <xhsmhed69ogjl.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 27 Aug 2024 22:29:50 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Chen Yu <yu.c.chen@...el.com>, Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, linux-kernel@...r.kernel.org, kprateek.nayak@....com,
 wuyun.abel@...edance.com, youssefesmat@...omium.org, tglx@...utronix.de,
 efault@....de
Subject: Re: [PATCH 12/24] sched/fair: Prepare exit/cleanup paths for
 delayed_dequeue

On 27/08/24 17:35, Chen Yu wrote:
> On 2024-08-14 at 07:53:30 +0200, Peter Zijlstra wrote:
>> On Wed, Aug 14, 2024 at 12:07:57AM +0200, Peter Zijlstra wrote:
>> > On Tue, Aug 13, 2024 at 11:54:21PM +0200, Peter Zijlstra wrote:
>> >
>> > Obviously I remember it right after hitting send...
>> >
>> > We've just done:
>> >
>> >    dequeue_task();
>> >    p->sched_class = some_other_class;
>> >    enqueue_task();
>> >
>> > IOW, we're enqueued as some other class at this point. There is no way
>> > we can fix it up at this point.
>>
>> With just a little more sleep than last night, perhaps you're right
>> after all. Yes we're on a different class, but we can *still* dequeue it
>> again.
>
> Not quite get this. If the old class is cfs, the task is in a rb-tree. And
> if the new class is rt then the task is in the prio list. Just wonder
> would the rt.dequeue break the data of rb-tree?
>

On a class change e.g. CFS to RT, __sched_setscheduler() would
dequeue_task() (take it out of the RB tree), change the class,
enqueue_task() (put it in the RT priolist).

Then check_class_changed()->switched_from_fair() happens, dequeue_task(),
and that takes it out of the RT priolist. At least that's the theory, since
that currently explodes...

> thanks,
> Chenyu