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
