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Date: Tue, 30 Jan 2018 14:15:31 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Galbraith <efault@gmx.de>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        LKML <linux-kernel@vger.kernel.org>, rjw@rjwysocki.net,
        srinivas.pandruvada@linux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle
 candidate and the basis for SIS
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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_.