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

[Peter Zijlstra <peterz@infradead.org>]

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.

> > But now that you made me look, intel_pstate_hwp_set() is horrible crap.
> > You should _never_ do things like:
> > 
> >   rdmsr_on_cpu()
> >   /* frob value */
> >   wrmsr_on_cpu()
> > 
> > That's insane.
> 
> I guess you mean it does too many IPIs?  Or that it shouldn't do any IPIs
> at all?

Yes, too many synchronous IPIs, which themselves are typically already
more expensive than the MSR access.

At one point I looked to getting rid of the *msr_on_cpu() crud entirely,
but there's just too much users out there I didn't feel like touching.

If you really care you can do async IPIs and do a custom serialization
that only waits when you do back-to-back things, which should be fairly
uncommon I'd think.