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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.
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