Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS | Date | Wed, 31 Jan 2018 10:22:49 +0100 |
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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.
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