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Wysocki" , Thomas Gleixner , Ingo Molnar , Giovanni Gherdovich , Catalin Marinas , Will Deacon , Valentin Schneider , Dietmar Eggemann , Sean Kelley , Pierre Gondois , , , Subject: Re: [PATCH v3 2/3] arch_topology: obtain cpu capacity using information from CPPC To: Ionela Voinescu References: <20220302180913.13229-1-ionela.voinescu@arm.com> <20220302180913.13229-3-ionela.voinescu@arm.com> <4283eacf-6eab-b2f5-07f2-d19fad134277@huawei.com> <477748c7-ae9e-712b-90ad-b04a6767a996@huawei.com> From: Yicong Yang Message-ID: <1a18588b-c1a1-246f-01ae-dd431c7812d8@huawei.com> Date: Fri, 11 Mar 2022 16:42:19 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: X-Originating-IP: [10.67.102.169] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To canpemm500009.china.huawei.com (7.192.105.203) X-CFilter-Loop: Reflected X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220311_004229_605963_1E097A9B X-CRM114-Status: GOOD ( 34.51 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel