linux-kernel - Re: [PATCH] x86,sched: On AMD EPYC set freq_max = max

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Date:   Thu, 21 Jan 2021 02:09:14 +0100
From:   Giovanni Gherdovich <ggherdovich@...e.cz>
To:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Jon Grimm <Jon.Grimm@....com>,
        Nathan Fontenot <Nathan.Fontenot@....com>,
        Yazen Ghannam <Yazen.Ghannam@....com>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Suthikulpanit Suravee <Suravee.Suthikulpanit@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pu Wen <puwen@...on.cn>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>, x86@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH] x86,sched: On AMD EPYC set freq_max = max_boost in
 schedutil invariant formula

On Thu, 2021-01-21 at 01:35 +0100, Giovanni Gherdovich wrote:
> Phoronix.com discovered a severe performance regression on AMD APYC
> introduced on schedutil [see link 1] by the following commits from v5.11-rc1
> 
>     commit 41ea667227ba ("x86, sched: Calculate frequency invariance for AMD systems")
>     commit 976df7e5730e ("x86, sched: Use midpoint of max_boost and max_P for frequency invariance on AMD EPYC")
> 
> Furthermore commit db865272d9c4 ("cpufreq: Avoid configuring old governors as
> default with intel_pstate") from v5.10 made it extremely easy to default to
> schedutil even if the preferred driver is acpi_cpufreq. Distros are likely to
> build both intel_pstate and acpi_cpufreq on x86, and the presence of the
> former removes ondemand from the defaults. This situation amplifies the
> visibility of the bug we're addressing.
> 
> [link 1] https://www.phoronix.com/scan.php?page=article&item=linux511-amd-schedutil&num=1
> 
> 1. PROBLEM DESCRIPTION   : over-utilization and schedutil
> 2. PROPOSED SOLUTION     : raise freq_max in schedutil formula
> 3. DATA TABLE            : image processing benchmark
> 4. ANALYSIS AND COMMENTS : with over-utilization, freq-invariance is lost

I've sent this patch twice by mistake, sorry about that.

In case it is of interest, I've shared some plots made while studying this
regression in a bugzilla entry:

  https://bugzilla.kernel.org/show_bug.cgi?id=211305


Thanks,
Giovanni