linux-kernel - Re: [PATCH v4] sched/freq: move call to cpufreq_update

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Date:   Fri, 15 Nov 2019 11:37:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Doug Smythies <dsmythies@...us.net>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sargun Dhillon <sargun@...gun.me>, Tejun Heo <tj@...nel.org>,
        Xie XiuQi <xiexiuqi@...wei.com>, xiezhipeng1@...wei.com,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

On Fri, Nov 15, 2019 at 11:18:00AM +0100, Vincent Guittot wrote:
> On Fri, 15 Nov 2019 at 10:55, Peter Zijlstra <peterz@...radead.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?
> 
> 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.

> > > 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.