Re: [PATCH 12/24] sched/fair: Prepare exit/cleanup paths for delayed_dequeue

Peter Zijlstra
2024-08-13T22:08:11Z

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