Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency invariant accounting

[Giovanni Gherdovich]

On Thu, 2019-10-03 at 20:31 -0700, Srinivas Pandruvada wrote:
> On Thu, 2019-10-03 at 20:05 +0200, Rafael J. Wysocki wrote:
> > On Wednesday, October 2, 2019 2:29:26 PM CEST Giovanni Gherdovich
> > wrote:
> > > From: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > > 
> > > intel_pstate has two operating modes: active and passive. In "active"
> > > mode, the in-built scaling governor is used and in "passive" mode, the
> > > driver can be used with any governor like "schedutil". In "active" mode
> > > the utilization values from schedutil is not used and there is a
> > > requirement from high performance computing use cases, not to readas
> > > well any APERF/MPERF MSRs.
> > 
> > Well, this isn't quite convincing.
> > 
> > In particular, I don't see why the "don't read APERF/MPERF MSRs" argument
> > applies *only* to intel_pstate in the "active" mode.  What about
> > intel_pstate in the "passive" mode combined with the "performance"
> > governor?  Or any other governor different from "schedutil" for that
> > matter?
> > 
> > And what about acpi_cpufreq combined with any governor different from
> > "schedutil"?
> > 
> > Scale invariance is not really needed in all of those cases right now
> > AFAICS, or is it?
> 
> Correct. This is just part of the patch to disable in active mode
> (particularly in HWP and performance mode). 
> 
> But this patch is 2 years old. The folks who wanted this, disable
> intel-pstate and use userspace governor with acpi-cpufreq. So may be
> better to address those cases too.

I disagree with "scale invariance is needed only by the schedutil governor";
the two other users are the CPU's estimated utilization in the wakeup path,
via cpu_util_without(), as well as the load-balance path, via cpu_util() which
is used by update_sg_lb_stats().

Also remember that scale invariance is applied to both PELT signals util_avg
and load_avg; schedutil uses the former but not the latter.

I understand Srinivas patch to disable MSR accesses during the tick as a
band-aid solution to address a specific use case he cares about, but I don't
think that extending this approach to any non-schedutil governor is a good
idea -- you'd be killing load balancing in the process.


Giovanni