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

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Date:	Fri, 12 Feb 2016 17:15:59 +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>,
	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 5:01 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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.

At least intel_pstate should be able to figure out which P-state to
use then on the APERF/MPERF basis.