Re: [PATCH] sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr_running

[K Prateek Nayak]

Hello Chenyu,

On 9/24/2024 6:40 PM, Chen Yu wrote:
> [..snip..]
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 225b31aaee55..2859fc7e2da2 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -1025,7 +1025,7 @@ static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >        /*
> > >         * The task has consumed its request, reschedule.
> > >         */
> > > -     return true;
> > > +     return (cfs_rq->nr_running > 1);
> >
> > Was there a strong reason why Peter decided to use "rq->nr_running"
> > instead of "cfs_rq->nr_running" with PREEMPT_SHORT in update_curr()?
> >
> > I wonder if it was to force a pick_next_task() cycle to dequeue a
> > possibly delayed entity
> >   but AFAICT, "cfs_rq->nr_running" should
> > account for the delayed entity still on the cfs_rq and perhaps the
> > early return in update_curr() can just be changed to use
> > "cfs_rq->nr_running". Not sure if I'm missing something trivial.
> 85e511df3cec changes
> if (cfs_rq->nr_running > 1)  resched
> to
> if (rq->nr_running == 1) not_resched
> which does lower the bar to trigger resched
>
> Yes, I think your proposal make sense, the resched should only
> be triggered between 2 cfs tasks,
> and the restore to update_deadline() is not needed, something like below
> in update_curr() could also work:
>
> if (cfs_rq->nr_running == 1)
>        return;

That seems better IMO unless there was a strong reason for the original
change to use rq->nr_running :)

> thanks,
> Chenyu

--
Thanks and Regards,
Prateek