Re: [PATCH v3 04/14 RESEND] cpufreq: Add Hardware P-State (HWP) driver

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Wed, May 10, 2023 at 04:19:57PM +0200, Jan Beulich wrote:
> On 10.05.2023 15:54, Jason Andryuk wrote:
> > On Mon, May 8, 2023 at 2:33 AM Jan Beulich <jbeulich@suse.com> wrote:
> >> On 05.05.2023 17:35, Jason Andryuk wrote:
> >>> On Fri, May 5, 2023 at 3:01 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>> The other issue is that if you select "hwp" as the governor, but HWP
> >>> hardware support is not available, then hwp_available() needs to reset
> >>> the governor back to the default.  This feels like a layering
> >>> violation.
> >>
> >> Layering violation - yes. But why would the governor need resetting in
> >> this case? If HWP was asked for but isn't available, I don't think any
> >> other cpufreq handling (and hence governor) should be put in place.
> >> And turning off cpufreq altogether (if necessary in the first place)
> >> wouldn't, to me, feel as much like a layering violation.
> > 
> > My goal was for Xen to use HWP if available and fallback to the acpi
> > cpufreq driver if not.  That to me seems more user-friendly than
> > disabling cpufreq.
> > 
> >             if ( hwp_available() )
> >                 ret = hwp_register_driver();
> >             else
> >                 ret = cpufreq_register_driver(&acpi_cpufreq_driver);
> 
> That's fine as a (future) default, but for now using hwp requires a
> command line option, and if that option says "hwp" then it ought to
> be hwp imo.

As a downstrem distribution, I'd strongly prefer to have an option that
would enable HWP when present and fallback to other driver otherwise,
even if that isn't the default upstream. I can't possibly require large
group of users (either HWP-having or HWP-not-having) to edit the Xen
cmdline to get power management working well.

If the meaning for cpufreq=hwp absolutely must include "nothing if HWP
is not available", then maybe it should be named cpufreq=try-hwp
instead, or cpufreq=prefer-hwp or something else like this?

> > If we are setting cpufreq_opt_governor to enter hwp_available(), but
> > then HWP isn't available, it seems to me that we need to reset
> > cpufreq_opt_governor when exiting hwp_available() false.
> 
> This may be necessary in the future, but shouldn't be necessary right
> now. It's not entirely clear to me how that future is going to look
> like, command line option wise.
> 
> Jan
> 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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