Re: [PATCH 4/4] sched/fair: Proportional newidle balance

Peter Zijlstra
2026-01-23T10:50:57Z

On Sun, Jan 18, 2026 at 03:46:22PM -0500, Mario Roy wrote:
> The patch "Proportional newidle balance" introduced a regression
> with Linux 6.12.65 and 6.18.5. There is noticeable regression with
> easyWave testing. [1]
>
> The CPU is AMD Threadripper 9960X CPU (24/48). I followed the source
> to install easyWave [2]. That is fetching the two tar.gz archives.

What is the actual configuration of that chip? Is it like 3*8 or 4*6
(CCX wise). A quick google couldn't find me the answer :/

> #!/bin/bash
> # CXXFLAGS="-O3 $CXXFLAGS" ./configure
> # make -j8
>
> trap 'rm -f *.ssh *.idx *.log *.sshmax *.time' EXIT
>
> OMP_NUM_THREADS=48 ./src/easywave \
>   -grid examples/e2Asean.grd -source examples/BengkuluSept2007.flt \
>   -time 1200
>
>
> Before results with CachyOS 6.12.63-2 and 6.18.3-2 kernels.

So the problem is that 6.12 -> 6.18 is an enormous amount of kernel
releases :/ This patch in particular was an effort to fix a regression
caused by:

155213a2aed4 ("sched/fair: Bump sd->max_newidle_lb_cost when newidle balance fails")

I'm thinking that if you revert all 4 patches of this series your
performance will be even worse?

Anyway, my guess is that somehow this benchmark likes doing newidle even
if it is often not successful. I'll see if I can reproduce this on one
of my machine, but that might take a little while.