Message-ID: <20201022203913.GJ92942@lorien.usersys.redhat.com>
Date:   Thu, 22 Oct 2020 16:39:13 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Colin Ian King <colin.king@...onical.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "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 Mailing List <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Gilles Muller <Gilles.Muller@...ia.fr>,
        Srinivas Pandruvada <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 09:32:55PM +0100 Mel Gorman wrote:
> On Thu, Oct 22, 2020 at 07:59:43PM +0200, Rafael J. Wysocki wrote:
> > > > Agreed. I'd like the option to switch back if we make the default change.
> > > > It's on the table and I'd like to be able to go that way.
> > > >
> > >
> > > Yep. It sounds chicken, but it's a useful safety net and a reasonable
> > > way to deprecate a feature. It's also useful for bug creation -- User X
> > > running whatever found that schedutil is worse than the old governor and
> > > had to temporarily switch back. Repeat until complaining stops and then
> > > tear out the old stuff.
> > >
> > > When/if there is a patch setting schedutil as the default, cc suitable
> > > distro people (Giovanni and myself for openSUSE).
> > 
> > So for the record, Giovanni was on the CC list of the "cpufreq:
> > intel_pstate: Use passive mode by default without HWP" patch that this
> > discussion resulted from (and which kind of belongs to the above
> > category).
> > 
> 
> Oh I know, I did not mean to suggest that you did not. He made people
> aware that this was going to be coming down the line and has been looking
> into the "what if schedutil was the default" question.  AFAIK, it's still
> a work-in-progress and I don't know all the specifics but he knows more
> than I do on the topic. I only know enough that if we flipped the switch
> tomorrow that we could be plagued with google searches suggesting it be
> turned off again just like there is still broken advice out there about
> disabling intel_pstate for usually the wrong reasons.
> 
> The passive patch was a clear flag that the intent is that schedutil will
> be the default at some unknown point in the future. That point is now a
> bit closer and this thread could have encouraged a premature change of
> the default resulting in unfair finger pointing at one company's test
> team. If at least two distos check it out and it still goes wrong, at
> least there will be shared blame :/
> 
> > > Other distros assuming they're watching can nominate their own victim.
> > 
> > But no other victims had been nominated at that time.
> 
> We have one, possibly two if Phil agrees. That's better than zero or
> unfairly placing the full responsibility on the Intel guys that have been
> testing it out.
>

Yes. I agree and we (RHEL) are planning to test this soon. I'll try to get
to it.  You can certainly CC me, please, athough I also try to watch for this
sort of thing on list. 


Cheers,
Phil

> -- 
> Mel Gorman
> SUSE Labs
> 

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