Re: [PATCH] sched/cfs: change initial value of runnable_avg

[Vincent Guittot]

On Thu, 25 Jun 2020 at 11:24, Holger HoffstÃtte
<holger@xxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >   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