Messages in this thread | | | Subject | Re: [PATCH v2 2/2] cpufreq: intel_pstate: Conditional frequency invariant accounting | From | Giovanni Gherdovich <> | Date | Fri, 04 Oct 2019 10:29:46 +0200 |
| |
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.
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
| |