Re: [PATCH sched_ext/for-6.12] sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()

Tejun Heo
2024-11-09T20:17:48Z

Hello, Peter.

On Fri, Sep 06, 2024 at 11:04:20AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 05, 2024 at 03:17:13PM -1000, Tejun Heo wrote:
> > On Thu, Sep 05, 2024 at 06:41:42AM -1000, Tejun Heo wrote:
> > > > @@ -12716,6 +12716,12 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
> > > > if (this_rq->cfs.h_nr_running && !pulled_task)
> > > > pulled_task = 1;
> > > >
> > > > + /*
> > > > + * We pulled a task, but it got stolen before we re-acquired rq->lock.
> > > > + */
> > > > + if (!this_rq->cfs.h_nr_running && pulled_task)
> > > > + pulled_task = 0;
> > > > +
> > >
> > > Lemme test that.
> >
> > Did a bit of testing and it seems like it's mostly coming from delayed
> > dequeue handling. pick_next_entity() does this:
> >
> > struct sched_entity *se = pick_eevdf(cfs_rq);
> > if (se->sched_delayed) {
> > dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
> > SCHED_WARN_ON(se->sched_delayed);
> > SCHED_WARN_ON(se->on_rq);
> > return NULL;
> > }
> >
> > rq->cfs.nr_running includes the number of delay dequeued tasks which aren't
> > really runnable, so it seems like balance_fair() saying yes and
> > pick_next_entity() then hitting a delayed task.
>
> Duh, yes.
>
> > Maybe the solution is
> > tracking the number of delayed ones and subtracting that from nr_running?
>
> That came up yesterday for something else as well. Let me see if I can
> make that happen.
>
>
> Anyway, I suppose you keep your patch for now until I've managed to sort
> this out.

This still triggers. I'm going to apply the workaround for now.

Thanks.

--
tejun