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

[Holger HoffstÃtte]

On 2020-06-25 11:56, Vincent Guittot wrote:
> 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 ?

/proc/loadavg

> > 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 ?

After wakeup the _baseline_ for loadavg seemed to be the last value before suspend,
not 0. The 7 then was the base loadavg for a _mostly idle machine_ (just reading
mail etc.), i.e. it never went below said baseline again, no matter the
_actual_ load.

Here's an image: https://imgur.com/a/kd2stqO

Before 02:00 last night the load was ~7 (compiled something), then all processes
were terminated and the machine was suspended. After wakeup the machine was mostly
idle (9am..11am), yet measured loadavg continued with the same value as before.
I didn't notice this right away since my CPU meter on the desktop didn't show any
*actual* activity (because there was none). The spike at ~11am is the revert/reboot.
After that loadavg became normal again, i.e. representative of the actual load,
even after suspend/resume cycles.
I suspend/resume every night and the only thing that changed recently was this
patch, so.. :)

-h