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

[Giovanni Gherdovich]

On Fri, 2019-10-04 at 10:29 +0200, Rafael J. Wysocki wrote:
> On Fri, Oct 4, 2019 at 10:24 AM Giovanni Gherdovich <ggherdovich@xxxxxxx> wrote:
> > 
> > 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().
> 
> OK, so there are reasons to run the scale invariance code which are
> not related to the cpufreq governor in use.
> 
> I wonder then why those reasons are not relevant for intel_pstate in
> the "active" mode.
> 
> > 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.
> 
> But that is also the case for intel_pstate in the "active" mode, isn't it?

Sure it is.

Now, what's the performance impact of loosing scale-invariance in PELT signals?
And what's the performance impact of accessing two MSRs at the scheduler tick
on each CPU?

I am sporting Srinivas' patch because he expressed the concern that the losses
don't justify the gains for a specific class of users (supercomputing),
although I don't fully like the idea (and arguably that should be measured).


Giovanni