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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Nov 15, 2019 at 10:55 AM Peter Zijlstra <peterz@infradead.org> 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?
>
> > 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.
>
> If anything, update_blocked_avgerages() should look at
> @done/others_have_blocked() to emit events for rt,dl,irq.
>
> So why are we making the scheduler code more ugly instead of fixing that
> driver?

I guess we could "fix" the driver by making it rate limit MSR writes
only, but I'm not sure if that would help.