linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Message-ID: <CAJZ5v0giM=CMkQN770TLv5wgdUQ2o0D152HFG6-nX-rr483G+A@mail.gmail.com>
Date:	Fri, 12 Feb 2016 17:01:20 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux PM list <linux-pm@...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>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

On Fri, Feb 12, 2016 at 3:10 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Feb 11, 2016 at 10:52:20AM -0800, Steve Muckle wrote:
>> On 02/11/2016 09:30 AM, Peter Zijlstra wrote:
>> >> My concern above is that pokes are guaranteed to keep occurring when
>> >> > there is only RT or DL activity so nothing breaks.
>> >
>> > The hook in their respective tick handler should ensure stuff is called
>> > sporadically and isn't stalled.
>>
>> But that's only true if the RT/DL tasks happen to be running when the
>> tick arrives right?
>>
>> Couldn't we have RT/DL activity which doesn't overlap with the tick? And
>> if no CFS tasks happen to be executing on that CPU, we'll never trigger
>> the cpufreq update. This could go on for an arbitrarily long time
>> depending on the periodicity of the work.
>
> Possible yes, but why do we care? Such a CPU would be so much idle that
> cpufreq doesn't matter one way or another, right?

Well, in theory you can get 50% or so of the time active in bursts
that happen to fit between ticks.  If we happen to do those in the
lowest P-state, we may burn more energy than necessary on platforms
where more idle is preferred.