Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

[Vincent Guittot]

On Wed, 28 Nov 2018 at 14:33, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi <patrick.bellasi@xxxxxxx> wrote:
> >
> > On 28-Nov 11:02, Peter Zijlstra wrote:
> > > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote:
> > >
> > > > Is there anything else that I should do for these patches ?
> > >
> > > IIRC, Morten mention they break util_est; Patrick was going to explain.
> >
> > I guess the problem is that, once we cross the current capacity,
> > strictly speaking util_avg does not represent anymore a utilization.
> >
> > With the new signal this could happen and we end up storing estimated
> > utilization samples which will overestimate the task requirements.
> >
> > We will have a spike in estimated utilization at next wakeup, since we
> > use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in
> > case we collect multiple samples above the current capacity.
>
> TBH I don't see how it's different from current implementation with a
> task that was scheduled on big core and now wakes up on little core.
> The util_est is overestimated as well.
>
> But I'm fine with adding your proposal on to on the patchset
s/on to on/on top of/

>
> >
> > So, a possible fix could be to avoid storing util_est samples if we
> > end up with a utilization above the current capacity.
> >
> > Something like:
> >
> > ----8<---
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index ac855b2f4774..93e0cf5d8a76 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3661,6 +3661,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
> >         if (!task_sleep)
> >                 return;
> >
> > +       /* Skip samples which do not represent an actual utilization */
> > +       if (unlikely(task_util(p) > capacity_of(task_cpu(p))))
> > +               return;
> > +
> >         /*
> >          * If the PELT values haven't changed since enqueue time,
> >          * skip the util_est update.
> > ---8<---
> >
> > Could that work ?
> >
> > Maybe using a new utility function to wrap the new check.
> >
> > --
> > #include <best/regards.h>
> >
> > Patrick Bellasi