Re: [REGRESSION][v6.17-rc1]sched/fair: Bump sd->max_newidle_lb_cost when newidle balance fails

[Hazem Mohamed Abuelfotoh]

>> Hi Chris,
>> 
>> During testing, we are seeing a ~6% performance regression with the 
>> upstream stable v6.12.43 kernel (And Oracle UEK 
>> 6.12.0-104.43.4.el9uek.x86_64 kernel) when running the Phoronix 
>> pts/apache benchmark with 100 concurrent requests [0].  The regression 
>> is seen with the following hardware:
>> 
>> PROCESSOR: Intel Xeon Platinum 8167M Core Count: 8 Thread Count: 16 
>> Extensions: SSE 4.2 + AVX512CD + AVX2 + AVX + RDRAND + FSGSBASE Cache 
>> Size: 16 MB Microcode: 0x1 Core Family: Cascade Lake
>> 
>> After performing a bisect, we found that the performance regression was 
>> introduced by the following commit:
>> 
>> Stable v6.12.43: fc4289233e4b ("sched/fair: Bump sd->max_newidle_lb_cost 
>> when newidle balance fails")
>> Mainline v6.17-rc1: 155213a2aed4 ("sched/fair: Bump 
>> sd->max_newidle_lb_cost when newidle balance fails")
>> 
>> Reverting this commit causes the performance regression to not exist.
>> 
>> I was hoping to get your feedback, since you are the patch author.  Do 
>> you think gathering any additional data will help diagnose this issue?

> Hi everyone,

> Peter, we've had a collection of regression reports based on this
> change, so it sounds like we need to make it less aggressive, or maybe
> we need to make the degrading of the cost number more aggressive?

> Joe (and everyone else who has hit this), can I talk you into trying the
> drgn from
> https://lore.kernel.org/lkml/2fbf24bc-e895-40de-9ff6-5c18b74b4300@xxxxxxxx/

> I'm curious if it degrades at all or just gets stuck up high.

Hi All,

We are also seeing 20-30% performance regression on Database workloads
specifically Cassandra & Mongodb across multiple hardware platforms. We
have seen the regression on v6.1.149 & v6.12.43 and we were able to
bisect the regression to 155213a2aed4 ("sched/fair: Bump 
> sd->max_newidle_lb_cost when newidle balance fails"). 

We were able to reproduce this regression on below AWS instance types:

- c7a.4xlarge (16 4th generation AMD EPYC processors + 32 GiB RAM)
- c7i.4xlarge (16 4th Generation Intel Xeon Scalable processors + 32GiB RAM)
- c7g.4xlarge (16 AWS Arm based Graviton3 processors + 32 GiB RAM)
- c8g.4xlarge (16 AWS Arm based Graviton4 processors + 32 GiB RAM)

We will try drgn from
https://lore.kernel.org/lkml/2fbf24bc-e895-40de-9ff6-5c18b74b4300@xxxxxxxx/
 and will let you know the results. Meanwhile and given the significant
impact, Should we revert this commit on latest mainline & on impacted stable 
branches to stop the bleeding until we have a permanent fix?

Thank you.

Hazem