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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20201022162943.GL32041@suse.de>
Date:   Thu, 22 Oct 2020 17:29:43 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 05:25:14PM +0200, Peter Zijlstra wrote:
> 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.
> 

I know Intel ran a lot of tests, no question about it and no fingers are
being pointed. I know I've had enough bugs patches tested with a battery
of tests on various machines and still ended up with bug reports :)

> > 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.
> 

I've seen cases where HWP had variable behaviour between CPU
generations. It was hard to quantify and/or figure out because HWP is a
black box.

> > 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.
> 

That's fair.

-- 
Mel Gorman
SUSE Labs