2017-05-09 6:16 GMT+08:00 Rafael J. Wysocki <[email protected]>: > 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 <[email protected]>: >> > > From: Rafael J. Wysocki <[email protected]> >> > > >> > > 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? Regards, Wanpeng Li
