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

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Message-ID: <20180516072929.GN12235@hirez.programming.kicks-ass.net>
Date:   Wed, 16 May 2018 09:29:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...e.de, lenb@...nel.org,
        rjw@...ysocki.net, mgorman@...hsingularity.net, x86@...nel.org,
        linux-pm@...r.kernel.org, viresh.kumar@...aro.org,
        juri.lelli@....com, linux-kernel@...r.kernel.org
Subject: Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional
 frequency invariant accounting

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.