linux-kernel - Re: [PATCH] cpufreq: schedutil: govern how frequently we change frequency with rate

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Date:   Wed, 15 Feb 2017 23:52:49 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] cpufreq: schedutil: govern how frequently we change frequency with rate_limit

On Wednesday, February 15, 2017 11:35:29 PM Rafael J. Wysocki wrote:
> On Wednesday, February 15, 2017 10:45:47 PM Viresh Kumar wrote:
> 
> First of all, [RFC] pretty please on things like this.
> 
> > For an ideal system (where frequency change doesn't incur any penalty)
> > we would like to change the frequency as soon as the load changes for a
> > CPU. But the systems we have to work with are far from ideal and it
> > takes time to change the frequency of a CPU. For many ARM platforms
> > specially, it is at least 1 ms. In order to not spend too much time
> > changing frequency, we have earlier introduced a sysfs controlled
> > tunable for the schedutil governor: rate_limit_us.
> > 
> > Currently, rate_limit_us controls how frequently we reevaluate frequency
> > for a set of CPUs controlled by a cpufreq policy. But that may not be
> > the ideal behavior we want.
> > 
> > Consider for example the following scenario. The rate_limit_us tunable
> > is set to 10 ms. The CPU has a constant load X and that requires the
> > frequency to be set to Y. The schedutil governor changes the frequency
> > to Y, updates last_freq_update_time and we wait for 10 ms to reevaluate
> > the frequency again. After 10 ms, the schedutil governor reevaluates the
> > load and finds it to be the same. And so it doesn't update the
> > frequency, but updates last_freq_update_time before returning. Right
> > after this point, the scheduler puts more load on the CPU and the CPU
> > needs to go to a higher frequency Z. Because last_freq_update_time was
> > updated just now, the schedutil governor waits for additional 10ms
> > before reevaluating the load again.
> > 
> > Normally, the time it takes to reevaluate the frequency is negligible
> > compared to the time it takes to change the frequency.
> 
> This should be "the time it takes to reevaluate the load is negligible
> relative to the time it takes to change the frequency" I suppose?
> 
> Specifically, the "to reevaluate the frequency" phrase is ambiguous.
> 
> > And considering
> > that in the above scenario, as we haven't updated the frequency for over
> > 10ms, we should have changed the frequency as soon as the load changed.
> 
> Why should we?
> 
> > This patch changes the way rate_limit_us is used, i.e. It now governs
> > "How frequently we change the frequency" instead of "How frequently we
> > reevaluate the frequency".
> 
> That's questionable IMO.

It actually changes the meaning of rate_limit_us, which may not be wrong in
principle, but really the question is what its meaning *should* be.

Thanks,
Rafael