linux-kernel - Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

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Message-ID: <4353713.vEOq6OHvJN@aspire.rjw.lan>
Date:   Wed, 31 Jan 2018 10:22:49 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

On Tuesday, January 30, 2018 2:15:31 PM CET Peter Zijlstra wrote:
> On Tue, Jan 30, 2018 at 12:57:18PM +0000, Mel Gorman wrote:
> > On Tue, Jan 30, 2018 at 12:50:54PM +0100, Peter Zijlstra wrote:
> 
> > > Not saying this patch is bad; but Rafael / Srinivas we really should do
> > > better. Why isn't cpufreq (esp. sugov) fixing this? HWP or not, we can
> > > still give it hints, and it looks like we're not doing that.
> > > 
> > 
> > I'm not sure if HWP can fix it because of the per-cpu nature of its
> > decisions. I believe it can only give the most basic of hints to hardware
> > like an energy performance profile or bias (EPP and EPB respectively).
> > Of course HWP can be turned off but not many people can detect that it's
> > an appropriate decision, or even desirable, and there is always the caveat
> > that disabling it increases the system CPU footprint.
> 
> IA32_HWP_REQUEST has "Minimum_Performance", "Maximum_Performance" and
> "Desired_Performance" fields which can be used to give explicit
> frequency hints. And we really _should_ be doing that.
> 
> Because, esp. in this scenario; a task migrating; the hardware really
> can't do anything sensible, whereas the OS _knows_.

But IA32_HWP_REQUEST is not a cheap MSR to write to.