Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely

[Wanpeng Li <kernellwp@gmail.com>]

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?

Regards,
Wanpeng Li