[Intel-gfx] [RFC] GPU-bound energy efficiency improvements for the intel_pstate driver (v2.99)

[Rafael J. Wysocki]

On Monday, May 11, 2020 11:01:41 PM CEST Francisco Jerez wrote:
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> Peter Zijlstra <peterz at infradead.org> writes:
> 
> > On Mon, Apr 27, 2020 at 08:22:47PM -0700, Francisco Jerez wrote:
> >> This addresses the technical concerns people brought up about my
> >> previous v2 revision of this series.  Other than a few bug fixes, the
> >> only major change relative to v2 is that the controller is now exposed
> >> as a new CPUFREQ generic governor as requested by Rafael (named
> >> "adaptive" in this RFC though other naming suggestions are welcome).
> >> Main reason for calling this v2.99 rather than v3 is that I haven't
> >> yet addressed all the documentation requests from the v2 thread --
> >> Will spend some time doing that as soon as I have an ACK (ideally from
> >> Rafael) that things are moving in the right direction.
> >>=20
> >> You can also find this series along with the WIP code for non-HWP
> >> platforms in this branch:
> >>=20
> >> https://github.com/curro/linux/tree/intel_pstate-vlp-v2.99
> >>=20
> >> Thanks!
> >>=20
> >> [PATCHv2.99 01/11] PM: QoS: Add CPU_SCALING_RESPONSE global PM QoS limit.
> >> [PATCHv2.99 02/11] drm/i915: Adjust PM QoS scaling response frequency ba=
> sed on GPU load.
> >> [PATCHv2.99 03/11] OPTIONAL: drm/i915: Expose PM QoS control parameters =
> via debugfs.
> >> [PATCHv2.99 04/11] cpufreq: Define ADAPTIVE frequency governor policy.
> >> [PATCHv2.99 05/11] cpufreq: intel_pstate: Reorder intel_pstate_clear_upd=
> ate_util_hook() and intel_pstate_set_update_util_hook().
> >> [PATCHv2.99 06/11] cpufreq: intel_pstate: Call intel_pstate_set_update_u=
> til_hook() once from the setpolicy hook.
> >> [PATCHv2.99 07/11] cpufreq: intel_pstate: Implement VLP controller stati=
> stics and target range calculation.
> >> [PATCHv2.99 08/11] cpufreq: intel_pstate: Implement VLP controller for H=
> WP parts.
> >> [PATCHv2.99 09/11] cpufreq: intel_pstate: Enable VLP controller based on=
>  ACPI FADT profile and CPUID.
> >> [PATCHv2.99 10/11] OPTIONAL: cpufreq: intel_pstate: Add tracing of VLP c=
> ontroller status.
> >> [PATCHv2.99 11/11] OPTIONAL: cpufreq: intel_pstate: Expose VLP controlle=
> r parameters via debugfs.
> >
> > What I'm missing is an explanation for why this isn't using the
> > infrastructure that was build for these kinds of things? The thermal
> > framework, was AFAIU, supposed to help with these things, and the IPA
> > thing in particular is used by ARM to do exactly this GPU/CPU power
> > budget thing.
> >
> > If thermal/IPA is found wanting, why aren't we improving that?
> 
> The GPU/CPU power budget "thing" is only a positive side effect of this
> series on some TDP-bound systems.  Its ultimate purpose is improving the
> energy efficiency of workloads which have a bottleneck on a device other
> than the CPU, by giving the bottlenecking device driver some influence
> over the response latency of CPUFREQ governors via a PM QoS interface.
> This seems to be completely outside the scope of the thermal framework
> and IPA AFAIU.
> 
> >
> > How much of that ADAPTIVE crud is actually intel_pstate specific? On a
> > (really) quick read it appears to me that much of the controller bits
> > there can be applied more generic, and thus should not be part of any
> > one governor.
> >
> 
> The implementation of that is intel_pstate-specific right now, but the
> basic algorithm could be made to work on any other governor in
> principle, which is why it is exposed as a generic CPUFREQ governor.  I
> don't care about taking out the generic CPUFREQ governor changes if you
> don't like them, and going back to some driver-specific means of turning
> it on and off (though Rafael might disagree with that).
> 
> > Specifically, I want to use sched_util as cpufreq governor and use the
> > intel_pstate as a passive driver.
> 
> Yeah, getting a similar optimization into the schedutil governor has
> been on my wish list for a while, but I haven't had the time to get very
> far on that except for a handful of hacks.  The intel_pstate handling is
> going to be necessary anyway in order to handle HWP systems gracefully,
> at least in the near future until schedutil becomes a viable alternative
> to intel_pstate in active mode on HWP systems.

FWIW, work is under way to make intel_pstate in the passive mode work on HWP
systems.

I have a prototype patch for that, but it can be improved.  I'll post a new
version of it for review, possibly next week.

Cheers!