Re: will-it-scale.per_process_ops -15% regression on v6.16-rc1

["D, Suneeth" <Suneeth.D@amd.com>]

Hello,

On 7/10/2025 5:21 PM, D, Suneeth wrote:
> Greetings,
> 
> We have observed ~(15-16)% regression on the will-it-scale-page_fault3 
> variant when the testcase was run in process mode on the kernel 
> v6.16-rc5. Further investigation showed that the regression was sparked 
> in v6.16-rc1 and has propagated all its way through v6.16-rc5.
> 
> Below are the test parameters and Machine configuration that have been 
> tested with:-
> 
> testcase: will-it-scale
> compiler: gcc-13
> test machine: 256 threads 1 sockets AMD EPYC 9754 128-Core Processor @ 
> 2.2GHz [Bergamo] with 258G memory
> Test params:
> 
>          nr_task: [1 8 128 192 256]
>      mode: process
>      test: page_fault3
>      cpufreq_governor: performance
> 
> stable version (v6.15)    %diff     per_process_ops    kernel_rc_ver
> ----------------------  -----   --------------- -------------
> 492057            -15%    416927        v6.16-rc1
> 492057            -16%    414140        v6.16-rc2
> 492057            -15%    419158        v6.16-rc3
> 492057            -15%    420476        v6.16-rc4
> 492057            -15%    416334        v6.16-rc5
> 
> Also had a run with latest stable
> 
> v6.15    %diff     v6.15.5
> -----   -----   -------
> 492057     1%    494990
> 
> 
> Recreation steps:
> 
> 1) git clone https://github.com/antonblanchard/will-it-scale.git
> 2) git clone https://github.com/intel/lkp-tests.git
> 3) cd will-it-scale && git apply 
> lkp-tests/programs/will-it-scale/pkg/will-it-scale.patch
> 4) make
> 5) python3 ./runtest.py page_fault3 32 process 0 0 1 8 128 192 256
> 
> NOTE: [5] is specific to machine's architecture. starting from 1 is the 
> array of no.of tasks that you'd wish to run the testcase which here is 
> no.cores per CCX, per NUMA node/ per Socket, nr_threads.
> 
> Currently bisection is under progress b/w v6.15 and v6.16-rc1 to spot 
> the culprit commit.
> 

The bisection got me landed onto 
7ac67301e82f02b77a5c8e7377a1f414ef108b84 as the culprit commit. This was 
the same commit that was causing a regression in lmbench3 
micro-benchmark as well earlier for which the detailed discussion can be 
found in [1].

[1] 
https://lore.kernel.org/lkml/f59ef632-0d11-4ae7-bdad-d552fe1f1d78@amd.com/

> Thanks & Regards,
> Suneeth D
>