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

From: Phil Auld <pauld@redhat.com>
To: Mel Gorman <mgorman@suse.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Colin Ian King <colin.king@canonical.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Giovanni Gherdovich <ggherdovich@suse.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Julia Lawall <julia.lawall@inria.fr>,
	Ingo Molnar <mingo@redhat.com>,
	kernel-janitors@vger.kernel.org,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Gilles Muller <Gilles.Muller@inria.fr>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Linux PM <linux-pm@vger.kernel.org>,
	Len Brown <len.brown@intel.com>
Subject: Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core
Date: Thu, 22 Oct 2020 20:39:13 +0000	[thread overview]
Message-ID: <20201022203913.GJ92942@lorien.usersys.redhat.com> (raw)
In-Reply-To: <20201022203255.GN32041@suse.de>

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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