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

[Josh Don]

Hey Chris,

On Mon, Oct 6, 2025 at 2:25 PM Chris Mason <clm@xxxxxxxx> wrote:
>
> On 10/6/25 4:23 PM, Joseph Salisbury wrote:
> > 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.
>
> -chris
>

Adding another +1 to seeing this regression (for us, it's with a low
level storage system).

Historically it has been difficult to tune newidle balance in a single
way to support multiple different types of workloads. I'm wondering if
it might make sense to put a sched feature on toggling the cost
scaling for newidle balance, or otherwise making the aggressiveness
tunable.

Best,
Josh