linux-kernel - Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data

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Message-ID: <20160310084331.GO6344@twins.programming.kicks-ass.net>
Date:	Thu, 10 Mar 2016 09:43:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Steve Muckle <steve.muckle@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data

On Thu, Mar 10, 2016 at 12:28:52AM +0100, Rafael J. Wysocki wrote:
> > [ I would not have chosen (1 + 1/n), but lets stick to that ]
> 
> Well, what would you choose then? :-)

1/p ; 0 < p < 1

or so. Where p then represents the percentile threshold where you want
to bump to the next freq.

> I think that should be
> 
>   next_freq = (1 + 1/n) * max_freq * util / max
> 
> (where max is the second argument of cpufreq_update_util) or the
> dimensions on both sides don't match.

Well yes, but so far we were treating util (and util_raw) as 0 < u < 1,
values, so already normalized against max.

But yes..

> > if we substitute (2) into (3) we get:
> >
> >                   = (1 + 1/n) * max_freq * util_raw * current_freq / max_freq
> >                   = (1 + 1/n) * current_freq * util_raw (4)
> >
> > Which gets you two formula with the same general behaviour. As (2) is
> > the only approximation of (1) we can make.
> 
> OK
> 
> So since utilization is not frequency invariant in the current
> mainline (or linux-next for that matter) AFAIC, I'm going to use the
> following in the next version of the schedutil patch series:
> 
>   next_freq = 1.25 * current_freq * util_raw / max
> 
> where util_raw and max are what I get from cpufreq_update_util().
> 
> 1.25 is for the 80% tipping point which I think is reasonable.

OK.