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

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Date:	Fri, 12 Feb 2016 11:53:45 -0500
From:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 12 February 2016 at 11:15, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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.

Speaking for the generic case, it would be great to make use of such
feedback counters for selecting the next freq request. Use (num of
cycles used/total cycles) to figure out %ON time for the CPU. I
understand its not the goal for this patch series, but in the future
if we can do this in your callbacks where possible, then I think we
will do better than Ondemand.

Regards,
Ashwin.

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