linux-kernel - Re: [PATCH 09/10 v2] sched/fair: Fix sched_can_stop

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Message-ID: <CAKfTPtAcqXB=5FF_dteucisiYcdtFoaKVbqGE+iK3nipX7je_w@mail.gmail.com>
Date: Mon, 2 Dec 2024 10:18:54 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org, 
	pauld@...hat.com, efault@....de, luis.machado@....com, 
	Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>
Subject: Re: [PATCH 09/10 v2] sched/fair: Fix sched_can_stop_tick() for fair tasks

On Fri, 29 Nov 2024 at 19:26, K Prateek Nayak <kprateek.nayak@....com> wrote:
>
> (+ Tejun, David)
>
> Hello Vincent,
>
> On 11/29/2024 9:47 PM, Vincent Guittot wrote:
> > We can't stop the tick of a rq if there are at least 2 tasks enqueued in
> > the whole hierarchy and not only at the root cfs rq.
> >
> > rq->cfs.nr_queued tracks the number of sched_entity at one level
> > whereas rq->cfs.h_nr_enqueued tracks all enqueued tasks in the
> > hierarchy.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >   kernel/sched/core.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 3571f91d4b0d..866a1605656c 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1341,7 +1341,7 @@ bool sched_can_stop_tick(struct rq *rq)
> >       if (scx_enabled() && !scx_can_stop_tick(rq))
> >               return false;
> >
> > -     if (rq->cfs.nr_queued > 1)
> > +     if (rq->cfs.h_nr_queued > 1)
>
> Perhaps we can move this fix to the beginning of the series and add:
>
> Fixes: 11cc374f4643b ("sched_ext: Simplify scx_can_stop_tick() invocation in sched_can_stop_tick()")
>
> before converting the h_nr_running to h_nr_queued  since prior to that
> commit, sched_can_stop_tick() used to check "rq->nr_running" and since
> we check the count of DL, RR, and FIFO tasks up above, it would have
> captured number of fair tasks running before sched-ext. That way the fix
> can be backported easily to LTS too. Thoughts?

Yes I can do that

>
> >               return false;
> >
> >       /*
>
> --
> Thanks and Regards,
> Prateek
>