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

From: Peter Zijlstra
Date: Mon Nov 21 2016 - 06:48:13 EST

On Mon, Nov 21, 2016 at 05:00:16PM +0530, Viresh Kumar wrote:
> 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.

Dunno.. there's of course the cost amortization, but by the time we've
reached sugov_should_update_freq() most of the 'expensive' parts have
already been done from the scheduler's POV and its once again down to
the driver.