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

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Date:   Wed, 12 Apr 2017 10:01:07 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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>,
        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

Hi Rafael,

On 11/04/17 23:23, Rafael J. Wysocki wrote:
> 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.
> 

Sorry, it took a bit of time to get results.

I didn't see big power/perf regressions on interactive type of workloads
running on Pixel phones. Youtube only showed a +~2% energy increase, but
I can't say for certain that is not in the noise.

My experiments also have your other two patches [1] applied.

So, yeah. I guess we could go ahead with this and see if we can improve
further in the future.

Thanks,

- Juri

[1] https://marc.info/?l=linux-kernel&m=149178351728004&w=2