linux-kernel - Re: [PATCH] sched/fair: avoid vruntime compensation for SCHED

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Date:   Thu, 20 Aug 2020 15:15:32 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiang Biao <benbjiang@...il.com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: avoid vruntime compensation for SCHED_IDLE task

On Thu, 20 Aug 2020 at 14:58, <peterz@...radead.org> wrote:
>
> On Thu, Aug 20, 2020 at 02:51:06PM +0200, Vincent Guittot wrote:
> > On Thu, 20 Aug 2020 at 14:00, Jiang Biao <benbjiang@...il.com> wrote:
> > >
> > > From: Jiang Biao <benbjiang@...cent.com>
> > >
> > > Vruntime compensation has been down in place_entity() to
> > > boot the waking procedure for fair tasks. There is no need to
> >
> > s/boot/boost/ ?
> >
> > > do that for SCHED_IDLE task actually.
> > >
> > > Not compensating vruntime for SCHED_IDLE task could make
> > > SCHED_IDLE task more harmless for normal tasks.
>
> This is rather week. It would be much better if there's some actual data
> to support this claim.
>
> > > Signed-off-by: Jiang Biao <benbjiang@...cent.com>
> > > ---
> > >  kernel/sched/fair.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 1a68a0536add..adff77676a0a 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4115,7 +4115,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> > >                 vruntime += sched_vslice(cfs_rq, se);
> > >
> > >         /* sleeps up to a single latency don't count. */
> > > -       if (!initial) {
> > > +       if (!initial && likely(!task_has_idle_policy(task_of(se)))) {
> >
> > What if se is not a task ?
>
> Then we very much need it, because it might have fair tasks inside. I
> suppose you could do something complicated with idle_h_nr_running, but
> is all that really worth the effort?

Not sure that Jiang is using cgroups otherwise he would have seen a
warning I think.
That's been said, not compensating the vruntime for a sched_idle task
makes sense for me. Even if that will only help for others task in the
same cfs_rq

>
> > >                 unsigned long thresh = sysctl_sched_latency;
> > >
> > >                 /*
> > > --
> > > 2.21.0
> > >