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

[Rafael J. Wysocki]

On Fri, Feb 12, 2016 at 5:53 PM, Ashwin Chaugule
<ashwin.chaugule@xxxxxxxxxx> wrote:
> On 12 February 2016 at 11:15, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>> On Fri, Feb 12, 2016 at 5:01 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>>> 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.
>>
>> 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.

Yes, we can do that at least in principle.  intel_pstate is a proof of that.

Thanks,
Rafael