linux-kernel - Re: cpufreq: intel_pstate: map utilization into the pstate range

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Date:   Tue, 21 Dec 2021 18:04:36 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Francisco Jerez <currojerez@...eup.net>
Cc:     Julia Lawall <julia.lawall@...ia.fr>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: cpufreq: intel_pstate: map utilization into the pstate range

On Sun, Dec 19, 2021 at 11:10 PM Francisco Jerez <currojerez@...eup.net> wrote:
>
> Julia Lawall <julia.lawall@...ia.fr> writes:
>
> > On Sat, 18 Dec 2021, Francisco Jerez wrote:

[cut]

> > I did some experiements with forcing different frequencies.  I haven't
> > finished processing the results, but I notice that as the frequency goes
> > up, the utilization (specifically the value of
> > map_util_perf(sg_cpu->util) at the point of the call to
> > cpufreq_driver_adjust_perf in sugov_update_single_perf) goes up as well.
> > Is this expected?
> >
>
> Actually, it *is* expected based on our previous hypothesis that these
> workloads are largely latency-bound: In cases where a given burst of CPU
> work is not parallelizable with any other tasks the thread needs to
> complete subsequently, its overall runtime will decrease monotonically
> with increasing frequency, therefore the number of instructions executed
> per unit of time will increase monotonically with increasing frequency,
> and with it its frequency-invariant utilization.

But shouldn't these two effects cancel each other if the
frequency-invariance mechanism works well?