Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class change for throttled task

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 22, 2025 at 08:49:43PM +0800, Aaron Lu wrote:
> On Thu, May 22, 2025 at 02:03:36PM +0200, Peter Zijlstra wrote:

> > This is asymmetric -- dequeue removes it from that throttle list, but
> > the corresponding enqueue will not add it back, what gives?
> > 
> > Because now we have:
> > 
> >  p->on_rq=1
> >  p->throttle_node on list
> > 
> > move_queued_task()
> >   deactivate_task()
> >     dequeue_task_fair()
> >       list_del_init(throttle_node)
> >     p->on_rq = 2
> > 
> >   activate_task()
> >     enqueue_task_fair()
> >       // nothing special, makes the thing runnable
> >     p->on_rq = 1;
> > 
> > and we exit with a task that is on-rq and not throttled ?!?
> >
> > Why is this? Are we relying on pick_task_fair() to dequeue it again and
> > fix up our inconsistencies? If so, that had better have a comment on.
> 
> Correct.

But would it not be better to have enqueue bail when we're trying to
enqueue an already throttled task into a throttled cfs_rq?

It seems a waste to do the actual enqueue, pick, dequeue when we
could've just avoided all that.

The immediate problem seems to be that you destroy the
task_is_throttled() state on dequeue, but surely that is trivially
fixable by not keeping that state in the list.