linux-kernel - Re: [PATCH] cpufreq: schedutil: add up/down frequency transition rate limits

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Date:   Mon, 21 Nov 2016 13:26:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <Juri.Lelli@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Robin Randhawa <robin.randhawa@....com>,
        Steve Muckle <smuckle.linux@...il.com>, tkjos@...gle.com,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] cpufreq: schedutil: add up/down frequency transition
 rate limits

On Mon, Nov 21, 2016 at 12:14:32PM +0000, Juri Lelli wrote:
> On 21/11/16 11:19, Peter Zijlstra wrote:

> > So no tunables and rate limits here at all please.
> > 
> > During LPC we discussed the rampup and decay issues and decided that we
> > should very much first address them by playing with the PELT stuff.
> > Morton was going to play with capping the decay on the util signal. This
> > should greatly improve the ramp-up scenario and cure some other wobbles.
> > 
> > The decay can be set by changing the over-all pelt decay, if so desired.
> > 
> 
> Do you mean we might want to change the decay (make it different from
> ramp-up) once for all, or maybe we make it tunable so that we can
> address different power/perf requirements?

So the limited decay would be the dominant factor in ramp-up time,
leaving the regular PELT period the dominant factor for ramp-down.

(Note that the decay limit would only be applied on the per-task signal,
not the accumulated signal.)

It could be an option, for some, to build the kernel with a PELT window
of 16ms or so (half its current size), this of course means regenerating
all the constants etc.. And this very much is a compile time thing.

We could fairly easy; if this is so desired; make the PELT window size a
CONFIG option (hidden by default).

But like everything; patches should come with numbers justifying them
etc..

> > Also, there was the idea of; once the above ideas have all been
> > explored; tying the freq ram rate to the power curve.
> > 
> 
> Yep. That's an interesting one to look at, but it might require some
> time.

Sure, just saying that we should resist knobs until all other avenues
have been explored. Never start with a knob.