linux-kernel - Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency invariant accounting

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Date:   Thu, 03 Oct 2019 20:31:30 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>, Len Brown <lenb@...nel.org>,
        x86@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <qperret@...rret.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency
 invariant accounting

On Thu, 2019-10-03 at 20:05 +0200, Rafael J. Wysocki wrote:
> On Wednesday, October 2, 2019 2:29:26 PM CEST Giovanni Gherdovich
> wrote:
> > From: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> > 
> > intel_pstate has two operating modes: active and passive. In
> > "active"
> > mode, the in-built scaling governor is used and in "passive" mode,
> > the driver can be used with any governor like "schedutil". In
> > "active"
> > mode the utilization values from schedutil is not used and there is
> > a requirement from high performance computing use cases, not to
> > readas well
> > any APERF/MPERF MSRs.
> 
> Well, this isn't quite convincing.
> 
> In particular, I don't see why the "don't read APERF/MPERF MSRs"
> argument
> applies *only* to intel_pstate in the "active" mode.  What about
> intel_pstate
> in the "passive" mode combined with the "performance" governor?  Or
> any other
> governor different from "schedutil" for that matter?
> 
> And what about acpi_cpufreq combined with any governor different from
> "schedutil"?
> 
> Scale invariance is not really needed in all of those cases right now
> AFAICS,
> or is it?
Correct. This is just part of the patch to disable in active mode
(particularly in HWP and performance mode). 

But this patch is 2 years old. The folks who wanted this, disable
intel-pstate and use userspace governor with acpi-cpufreq. So may be
better to address those cases too.

> 
> So is the real concern that intel_pstate in the "active" mode reads
> the MPERF
> and APERF MSRs by itself and that kind of duplicates what the scale
> invariance
> code does and is redundant etc?
It is redundant in non-HWP mode. In HWP and performance (active mode)
we don't use atleast at this time.

Thanks
Srinivas