Messages in this thread | | | Subject | Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency invariant accounting | From | Giovanni Gherdovich <> | Date | Mon, 07 Oct 2019 10:33:50 +0200 |
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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@suse.cz> 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@linux.intel.com> > > > > > > > > > > > > > > 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
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