Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 4 Oct 2019 10:28:01 +0200 | Subject | Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency invariant accounting |
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On Fri, 4 Oct 2019 at 10:24, 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(). > > Also remember that scale invariance is applied to both PELT signals util_avg > and load_avg; schedutil uses the former but not the latter.
You have been quicker than me to reply. I was about to say the exact same things. scale invariance also helps the scheduler in task placement by stabilizing the metrics whatever the running frequency so a task will not be seen as a big task just because of a CPU running at lower frequency
> > 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
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