linux-kernel - Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

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Message-ID: <20201022152514.GJ2611@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Oct 2020 17:25:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Giovanni Gherdovich <ggherdovich@...e.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Ingo Molnar <mingo@...hat.com>,
        kernel-janitors@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Gilles Muller <Gilles.Muller@...ia.fr>,
        srinivas.pandruvada@...ux.intel.com,
        Linux PM <linux-pm@...r.kernel.org>,
        Len Brown <len.brown@...el.com>
Subject: Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core

On Thu, Oct 22, 2020 at 03:52:50PM +0100, Mel Gorman wrote:

> There are some questions
> currently on whether schedutil is good enough when HWP is not available.

Srinivas and Rafael will know better, but Intel does run a lot of tests
and IIRC it was found that schedutil was on-par for !HWP. That was the
basis for commit:

  33aa46f252c7 ("cpufreq: intel_pstate: Use passive mode by default without HWP")

But now it turns out that commit results in running intel_pstate-passive
on ondemand, which is quite horrible.

> There was some evidence (I don't have the data, Giovanni was looking into
> it) that HWP was a requirement to make schedutil work well.

That seems to be the question; Rafael just said the opposite.

> For distros, switching to schedutil by default would be nice because
> frequency selection state would follow the task instead of being per-cpu
> and we could stop worrying about different HWP implementations but it's

s/HWP/cpufreq-governors/ ? But yes.

> not at the point where the switch is advisable. I would expect hard data
> before switching the default and still would strongly advise having a
> period of time where we can fall back when someone inevitably finds a
> new corner case or exception.

Which is why I advocated to make it 'difficult' to use the old ones and
only later remove them.

> For reference, SLUB had the same problem for years. It was switched
> on by default in the kernel config but it was a long time before
> SLUB was generally equivalent to SLAB in terms of performance.

I remember :-)