linux-kernel - Re: [PATCH 1/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect to reduce number of frequency changes

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Date:   Thu, 22 Sep 2016 19:54:48 +0200
From:   Andreas Herrmann <aherrmann@...e.com>
To:     Stratos Karafotis <skarafotis@...il.com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stratos Karafotis <stratosk@...aphore.gr>,
        Thomas Renninger <trenn@...e.com>
Subject: Re: [PATCH 1/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect
 to reduce number of frequency changes

On Mon, Sep 19, 2016 at 10:39:18PM +0300, Stratos Karafotis wrote:
> On Mon, Sep 19, 2016 at 7:16 PM, Andreas Herrmann <aherrmann@...e.com> wrote:
> > On Fri, Sep 16, 2016 at 09:58:42PM +0300, Stratos Karafotis wrote:
> >> Hi,
> >>
> >> [ I 'm resending this message, because I think some recipients didn't receive
> >> it. ]
> >>
> >> On 16/09/2016 12:47 μμ, Andreas Herrmann wrote:
> >> > On Wed, Sep 07, 2016 at 10:32:01AM +0530, Viresh Kumar wrote:
> >> >> On 01-09-16, 15:21, Andreas Herrmann wrote:
> >> >>> On Mon, Aug 29, 2016 at 11:31:53AM +0530, Viresh Kumar wrote:

  ---8<---

> >> > It seems that the decision how to best map load values to target
> >> > frequencies is kind of hardware specific.
> >> >
> >> > Maybe a solution to this is that the cpufreq driver should be able to
> >> > provide a mapping function to overwrite the current default
> >> > calculation.

FYI, I've created new patches to address the issue.

First one will be to introduce a map_load_to_freq function. The
default being what commit 6393d6 introduced (no deadband).  Second
patch will than introduce a specific function for pcc-cpufreq to fall
back to what was used before commit 6393d6.

I just want to assemble gathered performance data and I am planning to
send those patches tomorrow.

> >> I'm not familiar with ppc-cpufreq drive but maybe patch 6393d6 just
> >> uncovered an "issue" that was already existed but only on higher loads.
> >>
> >> Because, with or without patch 6393d6, if the specific CPU doesn't
> >> use a frequency table, there will many frequency transitions in
> >> higher loads too. I believe, though, that the side effect it's smaller
> >> in higher frequencies because CPUs tend to work on lowest and highest
> >> frequencies.
> >
> > Might be. I didn't test this specifically.

Hopefully I'll also find time to gather some ftrace data wrt this.

> >> What about a patch in ppc-cpufreq driver that permits frequency
> >> changes only in specific steps and not in arbitrary values?
> >
> > Which steps would you use? What scheme would be universal usable for
> > all affected system using this driver?
> 
> Just an idea. I would split the frequency range (max_freq - min_freq)
> into ~10 steps. But I'm not familiar with the affected systems and
> of course I can't prove this is an ideal approach.

I've modified the pcc-cpufreq specific map_load_to_freq function to do
just that (map load values to 10 discrete frequency values) instead of
falling back to the deadband (pre-commit-6393d6-version).

Unfortunately this resulted in lower performance compared to
pre-commit-6393d6-version.

> > I had played with an approach to only make use of min_freq and
> > max_freq which eventually didn't result in better performance
> > in comparison to code before commit 6393d6.
> 
> Regards,
> Stratos


Regards,

Andreas