Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

[Vincent Guittot]

On Fri, 15 Nov 2019 at 12:59, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> Le Friday 15 Nov 2019 Ã 11:29:03 (+0100), Vincent Guittot a Ãcrit :
> > On Fri, 15 Nov 2019 at 11:18, Vincent Guittot
> > <vincent.guittot@xxxxxxxxxx> wrote:
> > >
> > > On Fri, 15 Nov 2019 at 10:55, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Nov 14, 2019 at 06:07:31PM +0100, Vincent Guittot wrote:
> > > > > update_cfs_rq_load_avg() calls cfs_rq_util_change() everytime pelt decays,
> > > > > which might be inefficient when cpufreq driver has rate limitation.
> > > > >
> > > > > When a task is attached on a CPU, we have call path:
> > > > >
> > > > > update_load_avg()
> > > > >   update_cfs_rq_load_avg()
> > > > >     cfs_rq_util_change -- > trig frequency update
> > > > >   attach_entity_load_avg()
> > > > >     cfs_rq_util_change -- > trig frequency update
> > > > >
> > > > > The 1st frequency update will not take into account the utilization of the
> > > > > newly attached task and the 2nd one might be discard because of rate
> > > > > limitation of the cpufreq driver.
> > > >
> > > > Doesn't this just show that a dumb rate limit in the driver is broken?
> > >
> > > But the rate limit may come from HW constraints that forces to wait
> > > let say 4ms or even 10ms between each frequency update.
> > >
> > > >
> > > > > update_cfs_rq_load_avg() is only called by update_blocked_averages()
> > > > > and update_load_avg() so we can move the call to
> > > > > cfs_rq_util_change/cpufreq_update_util() into these 2 functions. It's also
> > > > > interesting to notice that update_load_avg() already calls directly
> > > > > cfs_rq_util_change() for !SMP case.
> > > > >
> > > > > This changes will also ensure that cpufreq_update_util() is called even
> > > > > when there is no more CFS rq in the leaf_cfs_rq_list to update but only
> > > > > irq, rt or dl pelt signals.
> > > >
> > > > I don't think it does that; that is, iirc the return value of
> > > > ___update_load_sum() is 1 every time a period lapses. So even if the avg
> > > > is 0 and doesn't change, it'll still return 1 on every period.
> > > >
> > > > Which is what that dumb rate-limit thing wants of course. But I'm still
> > > > thinking that it's stupid to do. If nothing changes, don't generate
> > > > events.
> > >
> > > When everything (irq, dl, rt, cfs) is 0, we don't generate events
> > > because update_blocked_averages is no more called because
> > > rq->has_blocked_load is clear
> > >
> > > With current implementation, if cfs is 0 but not irq, dl or rt, we
> > > don't call cpufreq_update_util because it is only called through cfs
> > >
> > > >
> > > > If anything, update_blocked_avgerages() should look at
> > > > @done/others_have_blocked() to emit events for rt,dl,irq.
> > >
> > > other_have_blocked can be set but no decay happened during the update
> > > and we don't need to call cpufreq_update_util
> > >
> > > >
> > > > So why are we making the scheduler code more ugly instead of fixing that
> > > > driver?
> >
> > Also, I think that calling cfs_rq_util_change in
> > attach_entity_load_avg is not optimal because the attach can happen at
> > a child level before it has been propagated down to root
> > So I'm working on trying to remove it from attach_entity_load_avg and
> > keep it in update_load_avg. this would help cleaning the ugly
> >
> > -       } else if (decayed && (flags & UPDATE_TG))
> > -               update_tg_load_avg(cfs_rq, 0);
> > +       } else if (decayed) {
> > +               cfs_rq_util_change(cfs_rq, 0);
> > +
> > +               if (flags & UPDATE_TG)
> > +                       update_tg_load_avg(cfs_rq, 0);
> > +       }
> >  }
> >
>
> we can also do this instead :
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d377a3f..550b6bc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3614,15 +3614,15 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>                  *
>                  * IOW we're enqueueing a task on a new CPU.
>                  */
> -               attach_entity_load_avg(cfs_rq, se, SCHED_CPUFREQ_MIGRATION);
> +               attach_entity_load_avg(cfs_rq, se, 0);
>                 update_tg_load_avg(cfs_rq, 0);
> +               decayed = 1;
>
> -       } else if (decayed) {
> -               cfs_rq_util_change(cfs_rq, 0);
> +       } else if (decayed && (flags & UPDATE_TG))
> +               update_tg_load_avg(cfs_rq, 0);
>
> -               if (flags & UPDATE_TG)
> -                       update_tg_load_avg(cfs_rq, 0);
> -       }
> +       if (decayed)
> +               cfs_rq_util_change(cfs_rq, 0);
>  }

Forget this ... It's not enough and will continue to generate spurious
call when task is at root domain

>
>
>
> >