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

[Vincent Guittot]

On Fri, 15 Nov 2019 at 11:37, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Nov 15, 2019 at 11:18:00AM +0100, Vincent Guittot 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.
>
> Sure, but then it can still remember the value passed in last and use
> that state later.
>
> It doesn't _have_ to completely discard values.

yes but it means that we run at the "wrong" frequency during this
period and also that the cpufreq must in this case set a kind of timer
to resubmit a new frequency change out of scheduler event

>
> > > > 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
>
> Aah.. Ok, let me look at this thing again.
>
> > > 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
>
> True.