Re: [PATCH V3 0/4] cpufreq: cppc: Add support for frequency invariance

Hi Qian,

On Thursday 24 Jun 2021 at 11:17:55 (-0400), Qian Cai wrote:

On 6/24/2021 6:48 AM, Ionela Voinescu wrote:

quoted

Not if the counters are implemented properly. The kernel considers that
both reference and delivered performance counters should stop or reset
during idle. The kernel would not account for idle itself.

If the reference performance counter does not stop during idle, while
the core performance counter (delivered) does stop, the behavior above
should be seen very often.

Qian, do you see these small delivered performance values often or
seldom?

Ionela, so I managed to upgrade the kernel on the system to today's
linux-next which suppose to include this series. The delivered perf
is now 280. However, scaling_min_freq (200 MHz) is not equal to
lowest_perf (100).

scaling_driver: acpi_cppc

                  ^^^^^^^^^
I suppose you mean "cppc-cpufreq"?

"acpi_cppc" is not a scaling driver option.

scaling_governor: schedutil

Is that normal because lowest_nonlinear_perf is 200? 

Yes, that is right : [1]

Also, on this pretty idle system, 158 of 160 CPUs are always running
in max freq (280 MHz). The other 2 are running in 243 and 213 MHz
according to scaling_cur_freq. Apparently, "schedutil" does not work
proper on this system. I am going to try other governors to narrow
down the issue a bit.

So your CPUs run at frequencies between 200MHz and 280MHz?
Based on your acpi_cppc information below I would have assumed 2GHz as
lowest nonlinear and 2.8GHz as nominal. The reason for this is that
according to the ACPI spec the frequency values in the _CPC objects are
supposed to be in MHz, so 2800 MHz for nominal frequency would be
2.8GHz.

When you try more governors, make sure to check out the difference
between scaling_cur_freq and cpuinfo_cur_freq at [2]. The first gives
you the frequency that the governor (schedutil) is asking for, while the
second is giving you the current frequency obtained from the counters.

So to check the actual frequency the cores are running at, please check
cpuinfo_cur_freq.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/cpufreq/cppc_cpufreq.c?h=v5.13-rc7#n296
[2] https://www.kernel.org/doc/Documentation/cpu-freq/user-guide.txt

Thanks,
Ionela.

FYI, here is the acpi_cppc registers reading:

/sys/devices/system/cpu/cpu0/acpi_cppc/feedback_ctrs
ref:160705801 del:449594095
/sys/devices/system/cpu/cpu0/acpi_cppc/highest_perf
300
/sys/devices/system/cpu/cpu0/acpi_cppc/lowest_freq
1000
/sys/devices/system/cpu/cpu0/acpi_cppc/lowest_nonlinear_perf
200
/sys/devices/system/cpu/cpu0/acpi_cppc/lowest_perf
100
/sys/devices/system/cpu/cpu0/acpi_cppc/nominal_freq
2800
/sys/devices/system/cpu/cpu0/acpi_cppc/nominal_perf
280
/sys/devices/system/cpu/cpu0/acpi_cppc/reference_perf
100
/sys/devices/system/cpu/cpu0/acpi_cppc/wraparound_time
18446744073709551615