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

[Rafael J. Wysocki]

On Fri, Feb 12, 2016 at 3:10 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.