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

[Giovanni Gherdovich]

On Fri, 2019-10-04 at 08:17 -0700, Srinivas Pandruvada wrote:
> On Fri, 2019-10-04 at 10:57 +0200, Giovanni Gherdovich wrote:
> > 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).
> > 
> 
> I understand there are other impact of the scale invariance like in
> deadline code, which I didn't see when I submitted this patch.
> You can drop this patch at this time if you like. I can poke HPC folks
> to test a released kernel.

Thanks Srinivas, in v3 I'll drop the tick_disable mechanism for now.


Giovanni