linux-kernel - Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting

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Message-ID: <1526491957.61700.53.camel@linux.intel.com>
Date:   Wed, 16 May 2018 10:32:37 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional
 frequency invariant accounting

On Wed, 2018-05-16 at 11:07 +0200, Rafael J. Wysocki wrote:
> On Wed, May 16, 2018 at 9:29 AM, Peter Zijlstra <peterz@...radead.org
> > wrote:
> > On Wed, May 16, 2018 at 09:16:40AM +0200, Peter Zijlstra wrote:
> > > On Tue, May 15, 2018 at 09:49:03PM -0700, Srinivas Pandruvada
> > > wrote:
> > > > 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
> > > > read
> > > > any APERF/MPERF MSRs. In this case no need to use CPU cycles
> > > > for
> > > > frequency invariant accounting by reading APERF/MPERF MSRs.
> > > > With this change frequency invariant account is only enabled in
> > > > "passive" mode.
> > > 
> > > WTH is active/passive? Is passive when we select performance
> > > governor?
> > 
> > Bah, I cannot read it seems. active is when we use the intel_pstate
> > governor and passive is when we use schedutil and only use
> > intel_pstate
> > as a driver.
> > 
> > > Also; you have to explain why using APERF/MPERF is bad in that
> > > case. Why
> > > do they care if we read those MSRs during the tick?
> > 
> > That still stands.. this needs to be properly explained.
> 
> I guess this is from the intel_pstate perspective only.
> 
> The active mode is only used with HWP, so intel_pstate doesn't look
> at
> the utilization (in any form) in the passive mode today.
> 
> Still, there are other reasons for PELT to be scale-invariant, so ...
Not sure about the use case in active mode other than dynamic HWP boost
later in this series. If any, I can remove this patch.