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

[Rafael J. Wysocki]

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?