linux-kernel - Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max

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Message-ID: <YA6YEK4/rjtPLdkG@hirez.programming.kicks-ass.net>
Date:   Mon, 25 Jan 2021 11:06:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Giovanni Gherdovich <ggherdovich@...e.cz>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        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>,
        Michael Larabel <Michael@...ronix.com>, x86@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max_boost
 in schedutil invariant formula

On Fri, Jan 22, 2021 at 09:40:38PM +0100, Giovanni Gherdovich wrote:
> 1. PROBLEM DESCRIPTION (over-utilization and schedutil)
> 
> The problem happens on CPU-bound workloads spanning a large number of cores.
> In this case schedutil won't select the maximum P-State. Actually, it's
> likely that it will select the minimum one.
> 
> A CPU-bound workload puts the machine in a state generally called
> "over-utilization": an increase in CPU speed doesn't result in an increase of
> capacity. The fraction of time tasks spend on CPU becomes constant regardless
> of clock frequency (the tasks eat whatever we throw at them), and the PELT
> invariant util goes up and down with the frequency (i.e. it's not invariant
> anymore).

>                                       v5.10          v5.11-rc4
>                                       ~~~~~~~~~~~~~~~~~~~~~~~~
> CPU activity (mpstat)                 80-90%         80-90%
> schedutil requests (tracepoint)       always P0      mostly P2
> CPU frequency (HW feedback)           ~2.2 GHz       ~1.5 GHz
> PELT root rq util (tracepoint)        ~825           ~450
> 
> mpstat shows that the workload is CPU-bound and usage doesn't change with

So I'm having trouble with calling a 80%-90% workload CPU bound, because
clearly there's a ton of idle time.