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: <4761571.IWmCqhUQku@aspire.rjw.lan>
Date:   Sun, 04 Feb 2018 09:38:47 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        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>
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

On Friday, February 2, 2018 3:54:24 PM CET Srinivas Pandruvada wrote:
> On Fri, 2018-02-02 at 12:00 +0100, Rafael J. Wysocki wrote:
> > On Thursday, February 1, 2018 2:18:12 PM CET Srinivas Pandruvada
> > wrote:
> > > 
> > > On Thu, 2018-02-01 at 10:11 +0100, Peter Zijlstra wrote:
> > > > 
> > > > On Thu, Feb 01, 2018 at 08:50:28AM +0100, Rafael J. Wysocki
> > > > wrote:
> > > > > 
> > > > > 
> > > > > On Wednesday, January 31, 2018 11:17:10 AM CET Peter Zijlstra
> > > > > wrote:
> > > > > > 
> > > > > > 
> > > > > > On Wed, Jan 31, 2018 at 10:22:49AM +0100, Rafael J. Wysocki
> > > > > > wrote:
> > > > > > > 
> > > > > > > 
> > > > > > > On Tuesday, January 30, 2018 2:15:31 PM CET Peter Zijlstra
> > > > > > > wrote:
> > > > > > > 
> > > > > > > > 
> > > > > > > > 
> > > > > > > > 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.
> > > > > > That just means we might need to throttle writing to it, like
> > > > > > it
> > > > > > already
> > > > > > does for the regular pstate (PERF_CTRL) msr in any case
> > > > > > (also, is
> > > > > > that a
> > > > > > cheap msr?)
> > > > > > 
> > > > > > Not touching it at all seems silly.
> > > > > OK
> > > > > 
> > > > > So what field precisely would you touch?  "desired"?  If so,
> > > > > does
> > > > > that actually
> > > > > guarantee anything to happen?
> > > > No idea, desired would be the one I would start with, it matches
> > > > with
> > > > the intent here. But I've no idea what our current HWP
> > > > implementation
> > > > actually does with it.
> > > Desired !=0 will disable HWP autonomous mode of frequency
> > > selection.
> > But I don't think it will just run at "desired" then, will it?
> HWP all are these hints only not a guarantee.
> There are totally different way HWP is handled in client an servers.
> If you set desired all heuristics they collected will be dumped, so
> they suggest don't set desired when you are in autonomous mode. If we
> really want a boost set the EPP. We know that EPP makes lots of
> measurable difference.

Yeah, we've been consistently told to use EPP rather than "desired" for hints
like that.