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

From: Viresh Kumar
Date: Mon Nov 21 2016 - 06:30:24 EST

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.