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

[Chen Yu]

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:
> > > 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.
> 
> 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?

thanks,
Chenyu