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

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Date:   Mon, 12 Jun 2017 14:33:03 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Lists linaro-kernel <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 Mon, Jun 12, 2017 at 5:44 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 10-06-17, 23:21, Joel Fernandes wrote:
>> 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.
>
> Right and that's why I said in the cover letter that we may want to revert the
> offending commit for the time being as the solutions provided here have too much
> dependency on the resolve_freq() callback.

So I've decided to revert that commit for 4.12.

Thanks,
Rafael