Re: [PATCH v3 2/3] arch_topology: obtain cpu capacity using information from CPPC

Yicong Yang
2022-03-11T08:42:24Z

On 2022/3/10 23:08, Ionela Voinescu wrote:
> On Thursday 10 Mar 2022 at 14:39:12 (+0800), Yicong Yang wrote:
>> On 2022/3/9 23:37, Ionela Voinescu wrote:
>>> Hi Yicong,
>>>
>>> On Wednesday 09 Mar 2022 at 18:21:30 (+0800), Yicong Yang wrote:
>>>> Hi Ionela,
> [..]
>>>> Out of curiosity, since we have raw capacity now on ACPI system, seems we
>>>> are able to scale the capacity with freq_factor now? looked into
>>>> register_cpufreq_notifier().
>>>>
>>>
>>> The freq_factor is only used for DT systems where one provides
>>> "capacity-dmips-mhz" in DT. This entry actually represents DMIPS/MHz.
>>>
>>> So the freq_factor, set to:
>>>
>>> per_cpu(freq_factor, cpu) = policy->cpuinfo.max_freq / 1000;
>>>
>>> is used to obtain the performance at the maximum frequency, basically
>>> DMIPS = (Dhrystone) million instructions per second, by multiplying this
>>> raw value from DT with the freq_factor. After this, all these value for
>>> each CPU type are normalized on a scale [0, 1024], resulting in what we
>>> call CPU capacity.
>>>
>>
>> Thanks for the illustration. I can understand what DT does as it's defined
>> clearly in [1]. The CPUs in the system may have different capacity-dmips-mhz
>> as well as frequency so we first obtain the DMIPS/MHz and then scaled it with
>> the max frequency.
>>
>>> For ACPI systems freq_factor will have the default value of 1 when we
>>> call topology_normalize_cpu_scale(), as the performance value obtained
>>> from _CPC is already representative for the highest frequency of the CPU
>>> and not performance/Hz as we get from DT. Therefore, we are not and
>>> should not use a freq_factor here.
>>>
>>> Hopefully I understood your question correctly.
>>>
>>
>> Seems we have different meaning of raw capacity on DT based and ACPI based system.
>> On DT based system it doesn't consider the max frequency of each CPU so we need
>> to take the frequency into account later. But on ACPI based system the max perf
>> has already take the max frequency into account and we don't need to consider
>> the frequency differences among the CPUs. If so, the comment [2] is no more
>> correct as we don't need to scale the capcity according to the frequnecy but
>> not because that we cannot get the raw cpu capacity on ACPI based system.
>>
>
> Correct! I've fixed that comment in v4 [1].
>
>> The CPUs on ACPI based system may also have different DMIPS and maximum frequency,
>> the same with the DT based system. Is it possible to keep consistence with
>> what DT does? As defined by the spec[3], the CPPC aims to "maintaining a performance
>> definition that backs a continuous, abstract, unit-less performance scale" and
>> "leaves the definition of the exact performance metric to the platform." So is it
>> possible we can also interpret it as "capacity-dmips-mhz"?
>
> I don't believe so because of:
>
> "Platforms must use the same performance scale for all processors in the
> system. On platforms with heterogeneous processors, the performance
> characteristics of all processors may not be identical. In this case, the
> platform must synthesize a performance scale that adjusts for differences
> in processors, such that any two processors running the same workload at
> the same performance level will complete in approximately the same time."
>
> So IMO, given this description, it's not appropriate to provide/use
> capacity-dmips-mhz "performance levels" in _CPC. To have a very simple
> example, let's assume we have a system with two CPUs of the same u-arch
> but one of them is clocked at 1GHz while the other is clocked at 2GHz
> (fixed frequency for both). If we are to run a theoretical benchmark on
> both we would get 50% on the first and 100% on the second (considering we
> normalize the performance scores on a scale [0, 100%]). So we could have
> 50 and 100 as highest performance levels in _CPC, if one uses a system
> wide performance scale as described in the specification.
>
> But if we convert those values to DMIPS/MHz we would get the same value
> for both CPUs. But if we provide this same value as highest performance
> level in _CPC for both we break the rule of "running the same workload
> at the same performance level will complete in approximately the same
> time."
>

Thanks for the clarification. That sounds reasonable to me. :)

>> Then to scale the cpu with max frequency provided by cpufreq, for example
>> cppc_cpufreq. I'm not sure I'm right and understand it correctly, please
>> correct me if I'm wrong.
>
> All frequency information on ACPI system is optional so even if one
> would ever want to do something like the above, one might not know what> is the maximum frequency of a CPU. I believe the tendency in recent

I doubt that. Even the frequency in the _CPC is optional, the kernel may
get the maximum frequency in other ways. On my platform it's gotten from
DMI, see cppc_cpufreq_perf_to_khz(). But I'm not sure for other cpufreq
drivers.

Thanks,
Yicong