On Mon, 2021-02-15 at 20:24 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> Commit 3c55e94c0ade ("cpufreq: ACPI: Extend frequency tables to cover
> boost frequencies") attempted to address a performance issue involving
> acpi-cpufreq, the schedutil governor and scale-invariance on x86 by
> extending the frequency tables created by acpi-cpufreq to cover the
> entire range of "turbo" (or "boost") frequencies, but that caused
> frequencies reported via /proc/cpuinfo and the scaling_cur_freq
> attribute in sysfs to change which may confuse users and monitoring
> tools.
>
> For this reason, revert the part of commit 3c55e94c0ade adding the
> extra entry to the frequency table and use the observation that
> in principle cpuinfo.max_freq need not be equal to the maximum
> frequency listed in the frequency table for the given policy.
>
> Namely, modify cpufreq_frequency_table_cpuinfo() to allow cpufreq
> drivers to set their own cpuinfo.max_freq above that frequency and
> change acpi-cpufreq to set cpuinfo.max_freq to the maximum boost
> frequency found via CPPC.
>
> This should be sufficient to let all of the cpufreq subsystem know
> the real maximum frequency of the CPU without changing frequency
> reporting.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=211305
> Fixes: 3c55e94c0ade ("cpufreq: ACPI: Extend frequency tables to cover boost frequencies")
> Reported-by: Matt McDonald <gardotd426@xxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>
> Michael, Giovanni,
>
> The fix for the EPYC performance regression that was merged into 5.11 introduced
> an undesirable side-effect by distorting the CPU frequency reporting via
> /proc/cpuinfo and scaling_cur_freq (see the BZ link above for details).
>
> The patch below is reported to address this problem and it should still allow
> schedutil to achieve desirable performance, because it simply sets
> cpuinfo.max_freq without extending the frequency table of the CPU.
>
> Please test this one and let me know if it adversely affects performance.
>
> Thanks!
Hello Rafael,
more extended testing confirms the initial feeling; performance with this
patch is mostly identical to vanilla v5.11. Tbench shows an improvement.
Thanks for the fix!
Tested-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
Results follow. The machine has two sockets with an AMD EPYC 7742 each.
The governor is always schedutil.
Ratios of time, lower is better:
v5.11 v5.11
vanilla patch
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
NASA Parallel Benchmarks w/ MPI 1.00 0.96
NASA Parallel Benchmarks w/ OpenMP 1.00 ~
dbench on XFS 1.00 ~
Linux kernel compilation 1.00 ~
git unit test suite 1.00 ~
Ratio of throughput, higher is better:
v5.11 v5.11
vanilla patch
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
tbench on localhost 1.00 1.09
Tilde (~): no change wrt baseline.
Giovanni
