Date:   Wed, 22 Mar 2017 18:04:25 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     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>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....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

On Tue, Mar 21, 2017 at 4:08 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> 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
> over the whole policy and if that turns out to be low, reducing the
> frequency for the policy most likely is a good idea anyway.  On
> systems with one CPU per policy, however, it may affect performance
> adversely and even lead to increased energy consumption in some cases.
>
> On those systems it may be addressed by taking another utilization
> metric into consideration, like whether or not the CPU whose
> frequency is about to be reduced has been idle recently, because if
> that's not the case, the CPU is likely to be busy in the near future
> and its frequency should not be reduced.
>
> To that end, use the counter of idle calls in the timekeeping code.
> Namely, make the schedutil governor look at that counter for the
> current CPU every time before its frequency is about to be reduced.
> If the counter has not changed since the previous iteration of the
> governor computations for that CPU, the CPU has been busy for all
> that time and its frequency should not be decreased, so if the new
> frequency would be lower than the one set previously, the governor
> will skip the frequency update.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Makes sense,

Reviewed-by: Joel Fernandes <joelaf@...gle.com>

Thanks,
Joel

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