linux-kernel - Re: [PATCH 2/3] cpufreq: schedutil: Fix selection algorithm while reducing frequency

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Date:   Sat, 10 Jun 2017 23:21:30 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linaro-kernel@...ts.linaro.org,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        John Ettedgui <john.ettedgui@...il.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH 2/3] cpufreq: schedutil: Fix selection algorithm while
 reducing frequency

On Sat, Jun 10, 2017 at 2:11 AM, Joel Fernandes <joelaf@...gle.com> wrote:
> On Fri, Jun 9, 2017 at 3:15 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> While reducing frequency if there are no frequencies available between
>> "current" and "next" calculated frequency, then the core will never
>> select the "next" frequency.
>>
>> For example, consider the possible range of frequencies as 900 MHz, 1
>> GHz, 1.1 GHz, and 1.2 GHz. If the current frequency is 1.1 GHz and the
>> next frequency (based on current utilization) is 1 GHz, then the
>> schedutil governor will try to set the average of these as the next
>> frequency (i.e. 1.05 GHz).
>>
>> Because we always try to find the lowest frequency greater than equal to
>> the target frequency, cpufreq_driver_resolve_freq() will end up
>> returning 1.1 GHz only. And we will not be able to reduce the frequency
>> eventually. The worst hit is the policy->min frequency as that will
>> never get selected after the frequency is increased once.
>
> But once utilization goes to 0, it will select the min frequency
> (because it selects lowest frequency >= target)?

Never mind my comment about util 0, I see the problem you mention.
However I feel that this entire series adds complexity all to handle
the case of a false cache-miss which I think might not be that bad,
and the tradeoff with complexity/readability of the code kind of
negates the benefit. That's just my opinion about it fwiw.

Thanks,
Joel