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

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Message-ID: <Zs6Rue1ebxWyjnLx@chenyu5-mobl2>
Date: Wed, 28 Aug 2024 10:55:53 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Valentin Schneider <vschneid@...hat.com>
CC: Peter Zijlstra <peterz@...radead.org>, <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 2024-08-27 at 22:29:50 +0200, Valentin Schneider wrote:
> 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...
>

I see, thanks for this explaination! I overlooked that the task is already on
rt priolist. I applied Peter's dequeue patch with minor fix and did not see
the warning[1] after several cycle test(previously it is 100% reproducible).

[1] https://lore.kernel.org/lkml/Zs2ZoAcUsZMX2B%2FI@chenyu5-mobl2/

thanks,
Chenyu