linux-kernel - Re: [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware

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Message-ID: <20200213132024.GP14897@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Feb 2020 14:20:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Douglas Raillard <douglas.raillard@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        qperret@...gle.com, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware

On Thu, Feb 13, 2020 at 11:55:32AM +0000, Douglas Raillard wrote:
> On 2/10/20 1:30 PM, Peter Zijlstra wrote:

> > So ARM64 will soon get x86-like power management if I read these here
> > patches right:
> > 
> >   https://lkml.kernel.org/r/20191218182607.21607-2-ionela.voinescu@arm.com
> > 
> > And I'm thinking a part of Rafael's concerns will also apply to those
> > platforms.
> 
> AFAIU there is an important difference: ARM64 firmware should not end up
> increasing frequency on its own, it should only cap the frequency. That
> means that the situation stays the same for that boost:
> 
> Let's say you let schedutil selecting a freq that is +2% more power
> hungry. That will probably not be enough to make it jump to the next
> OPP, so you end up not boosting. Now if there is a firmware that decides
> for some reasons to cap frequency, it will be a similar situation.

The moment you give out OPP selection to a 3rd party (be it firmware or
a micro-controller) things are uncertain at best anyway.

Still, in general, if you give it higher input, it tends to at least
consider going faster -- which might be all you can ask for...

So I'm not exactly seeing what your argument is here.

> > Right, so the condition 'util_avg > util_est' makes sense to trigger
> > some sort of boost off of.
> > 
> > What kind would make sense for these platforms? One possibility would be
> > to instead of frobbing the energy margin, as you do here, to frob the C
> > in get_next_freq().
> 
> If I'm correct, changing the C value would be somewhat similar to the
> relative boosting I had in a previous version. Maybe adding a fixed
> offset would give more predictable results as was discussed with Vincent
> Guittot. In any case, it would change the perceived util (like iowait
> boost).

It depends a bit on what you change C into. If we do something trivial
like:
		1.25 ; !(util_avg > util_est)
	C := {
		2    ;  (util_avg > util_est)

ie. a binary selection of constants, then yes, I suppose that is the
case.

But nothing stops us from making it more complicated; or having it
depend on the presence of EM data.

> > (I have vague memories of this being proposed earlier; it also avoids
> > that double OPP iteration thing complained about elsewhere in this
> > thread if I'm not mistaken).
> 
> It should be possible to get rid of the double iteration mentioned by
> Quentin. Choosing to boost the util or the energy boils down to:
> 
> 1) If you care more about predictable battery life (or energy bill) than
> predictability of the boost feature, EM should be used.
> 
> 2) If you don't have an EM or you care more about having a predictable
> boost for a given workload, use util (or disable that boost).
> 
> The rational is that with 1), you will get a different speed boost for a
> given workload depending on the other things executing at the same time,
> as the speed up is not linear with the task-related metric (util -
> util_est). If you are already at high freq because of another workload,
> the speed up will be small because the next 100Mhz will cost much more
> than the same +100Mhz delta starting from a low OPP.

It's just that I'm not seeing how 1 actually works or provides that more
predictable battery life I suppose. We have this other sub-thread to
argue about that :-)

> > That is; I'm thinking it is important (esp. now that we got frequency
> > invariance sorted for x86), to have this patch also work for !EM
> > architectures (as those ARM64-AMU things would be).
> 
> For sure, that feature is supposed to help in cases that would be
> impossible to pinpoint with hardware, since it has to know what tasks
> execute.

OK, so I'm thinking we're agreeing that it would be good to have this
support !EM systems too.