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

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

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On 2025.12.03 08:45 Christian Loehle wrote:
> On 12/3/25 16:18, Harshvardhan Jha wrote:
>> Hi there,
>> 
>> While running performance benchmarks for the 5.15.196 LTS tags , it was
>> observed that several regressions across different benchmarks is being
>> introduced when compared to the previous 5.15.193 kernel tag. Running an
>> automated bisect on both of them narrowed down the culprit commit to:
>> - 5666bcc3c00f7 Revert "cpuidle: menu: Avoid discarding useful
>> information" for 5.15
>> 
>> Regressions on 5.15.196 include:
>> -9.3% : Phoronix pts/sqlite using 2 processes on OnPrem X6-2
>> -6.3% : Phoronix system/sqlite on OnPrem X6-2
>> -18%  : rds-stress -M 1 (readonly rdma-mode) metrics with 1 depth & 1
>> thread & 1M buffer size on OnPrem X6-2
>> -4 -> -8% : rds-stress -M 2 (writeonly rdma-mode) metrics with 1 depth &
>> 1 thread & 1M buffer size on OnPrem X6-2
>> Up to -30% : Some Netpipe metrics on OnPrem X5-2
>> 
>> The culprit commits' messages mention that these reverts were done due
>> to performance regressions introduced in Intel Jasper Lake systems but
>> this revert is causing issues in other systems unfortunately. I wanted
>> to know the maintainers' opinion on how we should proceed in order to
>> fix this. If we reapply it'll bring back the previous regressions on
>> Jasper Lake systems and if we don't revert it then it's stuck with
>> current regressions. If this problem has been reported before and a fix
>> is in the works then please let me know I shall follow developments to
>> that mail thread.
>
> The discussion regarding this can be found here:
> https://lore.kernel.org/lkml/36iykr223vmcfsoysexug6s274nq2oimcu55ybn6ww4il3g3cv@cohflgdbpnq7/
> we explored an alternative to the full revert here:
> https://lore.kernel.org/lkml/4687373.LvFx2qVVIh@rafael.j.wysocki/
> unfortunately that didn't lead anywhere useful, so Rafael went with the
> full revert you're seeing now.
>
> Ultimately it seems to me that this "aggressiveness" on deep idle tradeoffs
> will highly depend on your platform, but also your workload, Jasper Lake
> in particular seems to favor deep idle states even when they don't seem
> to be a 'good' choice from a purely cpuidle (governor) perspective, so
> we're kind of stuck with that.
>
> For teo we've discussed a tunable knob in the past, which comes naturally with
> the logic, for menu there's nothing obvious that would be comparable.
> But for teo such a knob didn't generate any further interest (so far).
>
> That's the status, unless I missed anything?

By reading everything in the links Chrsitian provided, you can see
that we had difficulties repeating test results on other platforms.

Of the tests listed herein, the only one that was easy to repeat on my
test server, was the " Phoronix pts/sqlite" one. I got (summary: no difference):

Kernel 6.18									Reverted			
pts/sqlite-2.3.0			menu rc4		menu rc1		menu rc1		menu rc3	
				performance		performance		performance		performance	
test	what			ave			ave			ave			ave	
1	T/C 1			2.147	-0.2%		2.143	0.0%		2.16	-0.8%		2.156	-0.6%
2	T/C 2			3.468	0.1%		3.473	0.0%		3.486	-0.4%		3.478	-0.1%
3	T/C 4			4.336	0.3%		4.35	0.0%		4.355	-0.1%		4.354	-0.1%
4	T/C 8			5.438	-0.1%		5.434	0.0%		5.456	-0.4%		5.45	-0.3%
5	T/C 12			6.314	-0.2%		6.299	0.0%		6.307	-0.1%		6.29	0.1%

Where:
T/C means: Threads / Copies
performance means: intel_pstate CPU frequency scaling driver and the performance CPU frequencay scaling governor.
Data points are in Seconds.
Ave means the average test result. The number of runs per test was increased from the default of 3 to 10.
The reversion was manually applied to kernel 6.18-rc1 for that test.
The reversion was included in kernel 6.18-rc3.
Kernel 6.18-rc4 had another code change to menu.c

In case the formatting gets messed up, the table is also attached.

Processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz, 6 cores 12 CPUs.
HWP: Enabled.
 
... Doug


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