linux-kernel - Re: [RFC/RFT][PATCH] cpufreq: schedutil: Reduce frequencies slower

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Date:   Tue, 11 Apr 2017 23:23:22 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        John <john.ettedgui@...il.com>
Subject: Re: [RFC/RFT][PATCH] cpufreq: schedutil: Reduce frequencies slower

On Thu, Mar 30, 2017 at 11:36 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> The schedutil governor reduces frequencies too fast in some
> situations which cases undesirable performance drops to
> appear.
>
> To address that issue, make schedutil reduce the frequency slower by
> setting it to the average of the value chosen during the previous
> iteration of governor computations and the new one coming from its
> frequency selection formula.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=194963
> Reported-by: John <john.ettedgui@...il.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>
> This addresses a practical issue, but one in the "responsiveness" or
> "interactivity" category which is quite hard to represent quantitatively.
>
> As reported by John in BZ194963, schedutil does not ramp up P-states quickly
> enough which causes audio issues to appear in his gaming setup.  At least it
> evidently is worse than ondemand in this respect and the patch below helps.
>
> The patch essentially repeats the trick added some time ago to the load-based
> P-state selection algorithm in intel_pstate, which allowed us to make it viable
> for performance-oriented users, and which is to reduce frequencies at a slower
> pace.
>
> The reason why I chose the average is because it is computationally cheap
> and pretty much the max reasonable slowdown and the idea is that in case
> there's something about to run that we don't know about yet, it is better to
> stay at a higher level for a while more to avoid having to get up from the floor
> every time.
>
> But technically speaking it is a filter. :-)
>
> So among other things I'm wondering if that leads to substantial increases in
> energy consumption anywhere.

I haven't seen any numbers indicating that, so since the issue
addressed by this patch is real, I'd like to go ahead with it for
4.12.

Thanks,
Rafael