Subject: Re: [RFC PATCH] Increase in idle power with schedutil

From: Peter Zijlstra <[email-masked]>
Date: Mon, 23 May 2016 11:24:16 +0200

On Sun, May 22, 2016 at 01:42:52PM -0700, Steve Muckle wrote:

> > So does it actually matter what the frequency is when you idle? Isn't
> > the whole thing clock gated anyway?
> > 
> > Because this seems to generate contradictory requirements, on the one
> > hand we want to stay idle as long as possible while on the other hand
> > you seem to want to clock down while idle, which requires not being
> > idle.
> > 
> > If it matters; should not your idle state muck explicitly set/restore
> > frequency?
> 
> AFAIK this is very platform dependent. Some will waste more power than
> others when a CPU idles above fmin due to things like resource (bus
> bandwidth, shared cache freq etc) voting.

Oh agreed, completely platform dependent. 'Luckily' all this cpuidle is
already very platform dependent.

> It is also true that there is power spent going to fmin (and then
> perhaps restoring the frequency when idle ends) which will be in part a
> function of how slow the frequency change operation is on that platform.

Agreed.

> I think Daniel Lezcano (added) was exploring the idea of having cpuidle
> drivers take the expected idle duration and potentially communicate to
> cpufreq to reduce the frequency depending on a platform-specific
> cost/benefit analysis.

Right; that's along the lines I was thinking. If the idle guestimate and
the idle QoS both allow (ie. it wins on power and doesn't violate
wake-up latency) muck with DVSF on the idle path.