Re: HWP vs "xenpm get-cpufreq-..."

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.01.2024 17:45, Jason Andryuk wrote:
> On Mon, Jan 15, 2024 at 11:04 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> I thought I'd try hwp on a SapphireRapids system, and of course - short
>> of easy other ways of seeing whether it actually has any apparent
>> effect - I've looked at "xenpm get-cpufreq-states" output. Just to find
>> that according to this, the system never enters any of the designated
>> Pn states (all CPUs are always claimed to be in P0), while at the same
>> time the P0 residencies remain in the hundreds of milliseconds. The
>> latter suggests CPUs do leave P0, yet it's then unclear what state they
>> are in during such time periods.
> 
> The `xenpm get-cpufreq-states` output may be stale under HWP.  If
> normally the acpi-cpufreq driver is updating them, they won't be
> updated under HWP.  I'll have to look into that more, but I guess they
> shouldn't be returned under HWP.

Likely - anything that conveys no real information (and hence is at
best confusing) would better be avoided.

>> I was also a little puzzled when comparing "xenpm get-cpufreq-para"
>> output with hwp against that with acpi-cpufreq: The latter lists a range
>> of 800MHz ... 2501MHz, while with hwp base is 2500MHz and max is 4800MHz.
>>
>> Do you have any explanation for these apparent anomalies?
> 
> The 1 in 2501 indicates the ACPI entry for the turbo frequencies.
> It's a limitation of the ACPI tables, AFAIU.  The tables weren't
> designed for turbo mode where the thermal constraints dynamically
> limit the maximum speed.  On systems I've looked at, there would also
> be a 2500MHz ACPI entry, which matches the HWP base.
> 
> The HWP 2500MHz & 4800MHz values are taken from cpuid output - I feel
> confident they are correct.  The output of `xenpm get-cpufreq-average`
> under HWP looks reasonable for scaling all the way up to 4800MHz
> (which can only be hit briefly before throttling down) - I think I've
> only seen ~4700MHz as a maximum in my use, and they it may drop to
> ~4200MHz for extended preiods.  Maybe try that command on your test
> system?

I'll try to remember to check that next time round.

Jan