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

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Date:   Mon, 12 Jun 2017 09:14:20 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Joel Fernandes <joelaf@...gle.com>
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 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.

-- 
viresh