RE: [PATCH v1 1/2] intel_pstate: Use the cpu load to determine the PercentPerformance

["Doug Smythies" <dsmythies@telus.net>]

On 2015.11.23 05:29 plongepe wrote:
> On 21/11/2015 17:21, Doug Smythies wrote:
>> On 2015.11.06 17:14 Srinivas Pandruvada wrote:
>> On Sat, 2015-11-07 at 02:09 +0100, Rafael J. Wysocki wrote:
>>> On Tuesday, November 03, 2015 10:27:19 AM Philippe Longepe wrote:
>>>> Aperf and Mperf counter are not enough to determine the Target P
>>>> -state
>>>> because they measure performance only when the targeted processor
>>>> is
>>>> in the C0 state (active state).
>>>> Because of that, we were computing the average P-state during the
>>>> last
>>>> period which can be very different from the average frequency
>>>> (or percentage of performance).
>>>>
>>>> As defined in the SDM (section 14.2), the PercentPerformance is
>>>> defined by:
>>>>
>>>> PercentPerformance = PercentBusy * (delta_aperf / delta_mperf);
>>>>
>>>> The PercentBusy (or load) can be estimated as the ratio of the
>>>> mperf
>>>> counter running at a constant frequency only during active periods
>>>> (C0)
>>>> and the time stamp counter running at the same frequency but also
>>>> during idle.
>>>>
>>>> So, PercentBusy = 100 * (delta_mperf / delta_tsc)
>>>>
>>>> and, PercentPerformance = 100 * (delta_mperf / delta_tsc) *
>>>> 				(delta_aperf / delta_mperf)
>>>> That can be simplified with:
>>>> PercentPerformance = 100 * (delta_aperf / delta_tsc)
>>
>> Yes, but the last time I tried to bring back actual load calculations
>> In a similar way it was pointed out that one should not do it this way.
>> [1] SDM also states:
>>
>> "Only the IA32_APERF/IA32_MPERF ratio is architecturally defined;
>> software should not attach meaning to the content of the individual of IA32_APERF or IA32_MPERF MSRs."
>
> Individual counters are not architecturally defined but their ratio is 
> defined.
> MERF and TSC counters are both counting at the same frequency (max non 
> turbo freq)
> but MPERF is counting only in active state (C0) and TSC is counting all 
> the time.
> So, delta_mperf/delta_tsc is representing the load.

I agree.
However, Intel should decide once and for all if mperf / tsc is allowed or not.
It was used one time, then it was deleted, then I tried to bring it back,
then I was told it wasn't allowed, now you are trying to bring it back.

... Doug