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

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Date:   Thu, 17 May 2018 18:56:37 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        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>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional
 frequency invariant accounting

On Thu, May 17, 2018 at 6:42 PM, Srinivas Pandruvada
<srinivas.pandruvada@...ux.intel.com> wrote:
> On Thu, 2018-05-17 at 18:16 +0200, Peter Zijlstra wrote:
>> On Thu, May 17, 2018 at 08:41:32AM -0700, Srinivas Pandruvada wrote:
>> > One more point to note. Even if we calculate some utilization based
>> > on
>> > the freq-invariant and arrive at a P-state, we will not be able to
>> > control any P-state in turbo region (not even as a cap) on several
>> > Intel processors using PERF_CTL MSRs.
>>
>> Right, but don't we need to set the PERF_CTL to max P in order to
>> access the turbo bins?
>
> Any PERF_CTL setting above what we call "Turbo Activation ratio" (which
> can be less than P1 read from platform info MSR) will do that on these
> systems (Most clients from Ivy bridge).
>
>>  So we still need to compute a P state, but as soon as we
>> reach max P, we're done.
>
> What will happen if we look at all core turbo as max and cap any
> utilization above this to 1024?

I was going to suggest that.

Otherwise, if we ever get (say) the max one-core turbo at any point
and the system only runs parallel workloads after that, it will appear
as underutilized.