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

[Rafael J. Wysocki]

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.