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

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Date:   Mon, 8 May 2017 13:15:49 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency
 of busy CPUs prematurely

2017-05-08 12:01 GMT+08:00 Viresh Kumar <viresh.kumar@...aro.org>:
> On 08-05-17, 11:49, Wanpeng Li wrote:
>> Hi Rafael,
>> 2017-03-22 7:08 GMT+08:00 Rafael J. Wysocki <rjw@...ysocki.net>:
>> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.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.
>
> That's why this code is changing it only for policy->cpu alone.

I see, thanks for the explanation.

Regards,
Wanpeng Li