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

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Message-ID: <20161121113016.GD10014@vireshk-i7>
Date:   Mon, 21 Nov 2016 17:00:16 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        Juri Lelli <Juri.Lelli@....com>,
        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 21-11-16, 12:12, Peter Zijlstra wrote:
> I think it should be replaced by a value provided by the driver. It
> makes sense to have a rate-limit in so far as that it doesn't make sense
> to try and program the hardware faster than it can actually change
> frequencies and/or have a programming cost amortization. And this very
> clearly is a driver specific thing.

We already have something called as transition_latency for that (though it isn't
used much currently).

> It however doesn't make sense to me to fudge with this in order to
> achieve ramp up/down differences.

So if a platform, for example, can do DVFS in say 100-500 us, then the scheduler
should try to re-evaluate frequency (and update it) after that short of a
period? Wouldn't that scheme waste lots of time doing just freq updates? And
that's the primary reason why cpufreq governors have some sort of sampling-rate
or rate-limit until now.

-- 
viresh