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

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Message-ID: <20240813220757.GA22760@noisy.programming.kicks-ass.net>
Date: Wed, 14 Aug 2024 00:07:57 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
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 Tue, Aug 13, 2024 at 11:54:21PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 13, 2024 at 02:43:47PM +0200, Valentin Schneider wrote:
> > On 27/07/24 12:27, Peter Zijlstra wrote:
> > > @@ -12817,10 +12830,26 @@ static void attach_task_cfs_rq(struct ta
> > >  static void switched_from_fair(struct rq *rq, struct task_struct *p)
> > >  {
> > >       detach_task_cfs_rq(p);
> > > +	/*
> > > +	 * Since this is called after changing class, this isn't quite right.
> > > +	 * Specifically, this causes the task to get queued in the target class
> > > +	 * and experience a 'spurious' wakeup.
> > > +	 *
> > > +	 * However, since 'spurious' wakeups are harmless, this shouldn't be a
> > > +	 * problem.
> > > +	 */
> > > +	p->se.sched_delayed = 0;
> > > +	/*
> > > +	 * While here, also clear the vlag, it makes little sense to carry that
> > > +	 * over the excursion into the new class.
> > > +	 */
> > > +	p->se.vlag = 0;
> > 
> > RQ lock is held, the task can't be current if it's ->sched_delayed; is a
> > dequeue_task() not possible at this point?  Or just not worth it?
> 
> Hurmph, I really can't remember why I did it like this :-(

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.

Perhaps I can use the sched_class::switching_to thing sched_ext will
bring.

For now, lets keep things as is. I'll look at things later