Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely | Date | Tue, 09 May 2017 01:01:43 +0200 |
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On Tuesday, May 09, 2017 06:36:14 AM Wanpeng Li wrote: > 2017-05-09 6:16 GMT+08:00 Rafael J. Wysocki <rjw@rjwysocki.net>: > > On Monday, May 08, 2017 09:31:19 AM Viresh Kumar wrote: > >> On 08-05-17, 11:49, Wanpeng Li wrote: > >> > Hi Rafael, > >> > 2017-03-22 7:08 GMT+08:00 Rafael J. Wysocki <rjw@rjwysocki.net>: > >> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > >> > > > >> > > The way the schedutil governor uses the PELT metric causes it to > >> > > underestimate the CPU utilization in some cases. > >> > > > >> > > That can be easily demonstrated by running kernel compilation on > >> > > a Sandy Bridge Intel processor, running turbostat in parallel with > >> > > it and looking at the values written to the MSR_IA32_PERF_CTL > >> > > register. Namely, the expected result would be that when all CPUs > >> > > were 100% busy, all of them would be requested to run in the maximum > >> > > P-state, but observation shows that this clearly isn't the case. > >> > > The CPUs run in the maximum P-state for a while and then are > >> > > requested to run slower and go back to the maximum P-state after > >> > > a while again. That causes the actual frequency of the processor to > >> > > visibly oscillate below the sustainable maximum in a jittery fashion > >> > > which clearly is not desirable. > >> > > > >> > > That has been attributed to CPU utilization metric updates on task > >> > > migration that cause the total utilization value for the CPU to be > >> > > reduced by the utilization of the migrated task. If that happens, > >> > > the schedutil governor may see a CPU utilization reduction and will > >> > > attempt to reduce the CPU frequency accordingly right away. That > >> > > may be premature, though, for example if the system is generally > >> > > busy and there are other runnable tasks waiting to be run on that > >> > > CPU already. > >> > > > >> > > This is unlikely to be an issue on systems where cpufreq policies are > >> > > shared between multiple CPUs, because in those cases the policy > >> > > utilization is computed as the maximum of the CPU utilization values > >> > > >> > Sorry for one question maybe not associated with this patch. If the > >> > cpufreq policy is shared between multiple CPUs, the function > >> > intel_cpufreq_target() just updates IA32_PERF_CTL MSR of the cpu > >> > which is managing this policy, I wonder whether other cpus which are > >> > affected should also update their per-logical cpu's IA32_PERF_CTL MSR? > >> > >> The CPUs share the policy when they share their freq/voltage rails and so > >> changing perf state of one CPU should result in that changing for all the CPUs > >> in that policy. Otherwise, they can't be considered to be part of the same > >> policy. > > > > To be entirely precise, this depends on the granularity of the HW interface. > > > > If the interface is per-logical-CPU, we will use it this way for efficiency > > reasons and even if there is some coordination on the HW side, the information > > on how exactly it works usually is limited. > > I check it on several Xeon servers on hand, however, I didn't find > /sys/devices/system/cpu/cpufreq/policyx/affected_cpus can affect more > than one logical cpu, so I guess most of Xeon servers are not support > shared cpufreq policy, then which kind of boxes support that?
On Intel the interface for performance scaling is per-logical-CPU in general.
Thanks, Rafael
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