RE: Performance regressions introduced via Revert "cpuidle: menu: Avoid discarding useful information" on 5.15 LTS

["Doug Smythies" <dsmythies@telus.net>]

On 2026.02.04 18:37 Sergey Senozhatsky wrote:
> On (26/02/04 16:45), Doug Smythies wrote:
>>>> What is "established" and "newer" for a stable kernel is quite handwavy
>>>> IMO but even here Sergey's regression report is a clear data point...
>>>
>>> Which wasn't known at the time commit 85975daeaa4d went in.
>>>
>>>> Your report is only restoring 5.15 (and others) performance to 5.15
>>>> upstream-ish levels which is within the expectations of running a stable
>>>> kernel. No doubt it's frustrating either way!
>>>
>>> That is a consequence of the time it takes for mainline changes to
>>> propagate to distributions (Chrome OS in this particular case) at
>>> which point they get tested on a wider range of systems.  Until that
>>> happens, it is not really guaranteed that the given change will stay
>>> in.
>>>
>>> In this particular case, restoring commit 85975daeaa4d would cause the
>>> same problems on the systems adversely affected by it to become
>>> visible again and I don't think it would be fair to say "Too bad" to
>>> the users of those systems.  IMV, it cannot be restored without a way
>>> to at least limit the adverse effect on performance.
>> 
>> I have been going over the old emails and the turbostat data again and again
>> and again.
>> 
>> I still do not understand how to breakdown Sergey's results into its
>> component contributions. I am certain there is power limit throttling
>> during the test, but have no idea to much or how little it contributes to the
>> differing results.
>> 
>> I think more work is needed to fully understand Sergey's test results from October.
>> I struggle with the dramatic test results difference of base=84.5 and revert=59.5
>> as being due to only the idle code changes.
>> 
>> That is why I keep asking for a test to be done with the CPU clock frequency limited
>> such that power limit throttling can not occur. I don't know what limit to use, but suggest
>> 2.2 GHZ to start with. Capture turbostat data with the tests. And record the test results.
>
>> @Sergey: are you willing to do the test?
>
> I can run tests, not immediately, though, but within some reasonable
> time frame.

Thanks.

> (I'll need some help with instructions/etc.)

From your turbostat data from October you are using the intel_pstate
CPU frequency scaling driver and the powersave CPU frequency scaling governor
and HWP enabled. Also your maximum CPU frequency is 3,300 MHz.

To limit the maximum CPU frequency to around 2,200 MHz do:

echo 67 |sudo tee /sys/devices/system/cpu/intel_pstate/max_perf_pct

Then run the tests acquiring turbostat logs the same way you did in October.

To restore the maximum CPU frequency afterwards do:

echo 100 |sudo tee /sys/devices/system/cpu/intel_pstate/max_perf_pct