Re: [PATCH 05/12] sched: Move sched_class::prio_changed() into the change pattern

[Peter Zijlstra]

On Tue, Jan 13, 2026 at 11:45:43AM +0100, Pierre Gondois wrote:
> Hello Prateek,
> 
> On 1/13/26 05:12, K Prateek Nayak wrote:
> > Hello Pierre,
> > 
> > On 1/13/2026 2:14 AM, Pierre Gondois wrote:
> > > Hello Peter,
> > > 
> > > It seems this patch:
> > > 6455ad5346c9 ("sched: Move sched_class::prio_changed() into the change pattern")
> > > is triggering the following warning:
> > > rq_pin_lock()
> > > \-WARN_ON_ONCE(rq->balance_callback && rq->balance_callback != &balance_push_callback);
> > Can you check if the following solution helps your case too:
> > https://lore.kernel.org/all/20260106104113.GX3707891@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > 
> I can still see the issue.
> It seems the task deadline is also updated in:
> sched_change_end()
> \-enqueue_task_dl()
>   \-enqueue_dl_entity()
>     \-setup_new_dl_entity()
>       \-replenish_dl_new_period()
> if the task's period finished.
> 
> So in sched_change_end(), the task priority (i.e. p->dl.deadline) is
> updated.
> This results in having an old_deadline earlier than the new p->dl.deadline.
> Thus the rq->balance_callback:
> 
> prio_changed_dl() {
> ...
> if (dl_time_before(old_deadline, p->dl.deadline))
>   deadline_queue_pull_task(rq);
> ...
> }

Hum... so this one is a little more tricky.

So the normal rules are that DEQUEUE_SAVE + ENQUEUE_RESTORE should be as
invariant as possible.

But what I think happens here is that at the point of dequeue we are
effectively ready to throttle/replenish, but we don't.

Then at enqueue, we do. The replenish changes the deadline and we're up
a creek.

Let me think about this for a bit...