linux-kernel - Re: [PATCH] sched/cfs: change initial value of runnable

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Date:   Thu, 25 Jun 2020 11:56:32 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        kernel test robot <rong.a.chen@...el.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Phil Auld <pauld@...hat.com>, Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] sched/cfs: change initial value of runnable_avg

On Thu, 25 Jun 2020 at 11:24, Holger Hoffstätte
<holger@...lied-asynchrony.com> wrote:
>
> On 2020-06-24 17:44, Vincent Guittot wrote:
> > Some performance regression on reaim benchmark have been raised with
> >    commit 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
> >
> > The problem comes from the init value of runnable_avg which is initialized
> > with max value. This can be a problem if the newly forked task is finally
> > a short task because the group of CPUs is wrongly set to overloaded and
> > tasks are pulled less agressively.
> >
> > Set initial value of runnable_avg equals to util_avg to reflect that there
> > is no waiting time so far.
> >
> > Fixes: 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group")
> > Reported-by: kernel test robot <rong.a.chen@...el.com>
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >   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 0424a0af5f87..45e467bf42fc 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -806,7 +806,7 @@ void post_init_entity_util_avg(struct task_struct *p)
> >               }
> >       }
> >
> > -     sa->runnable_avg = cpu_scale;
> > +     sa->runnable_avg = sa->util_avg;
> >
> >       if (p->sched_class != &fair_sched_class) {
> >               /*
> >
>
> Something is wrong here. I woke up my machine from suspend-to-RAM this morning
> and saw that a completely idle machine had a loadavg of ~7. According to my

Just to make sure: Are you speaking about loadavg that is output by
/proc/loadavg or load_avg which is the PELT load ?
The output of /proc/loadavg hasn't any link with runnable_avg. The 1st
one monitors nr_running at 5 sec interval whereas the other one is a
geometrics series of the weight of runnable tasks with a half time of
32ms

> monitoring system this happened to be the loadavg right before I suspended.
> I've reverted this, rebooted, created a loadavg >0, suspended and after wake up
> loadavg again correctly ranges between 0 and whatever, as expected.

I'm not sure to catch why ~7 is bad compared to correctly ranges
between 0 and whatever. Isn't ~7 part of the whatever ?

Vincent

>
> -h