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

Mario Roy
2026-01-27T04:15:59Z

I tried the Stress-NG socket activity test. Plus prefer-idle-core patch.
The patch is about mindfulness for limited CPU saturation testing.

AMD Ryzen Threadripper 9960X CPU (24/48)

Bogo operations/second, More is better
A        B        C        D        E        F
SocketAct    12128.7  13907.6  12377.7  10551.7  12158.7  11842.2
SocketAct24  64553.3  20072.0  67018.7  62182.3  18133.5  66756.6
SocketAct15  49206.3  22170.7  57038.7  44077.6  19884.1  56727.5
SocketAct10  35263.5  20140.3  40092.1  33040.3  19701.6  41346.3

The kernels are built with clang without LTO/AutoFDO

A. 6.19-rc7 next_buddy ena with sched/fair: Proportional newidle balance
B. 6.19-rc7 next_buddy ena without sched/fair: Proportional newidle balance
C. 6.19-rc7 next_buddy ena without sched regression; with prefer-idle-core

D. 6.19-rc7 next_buddy dis with sched/fair: Proportional newidle balance
E. 6.19-rc7 next_buddy dis without sched/fair: Proportional newidle balance
F. 6.19-rc7 next_buddy dis without sched regression; with prefer-idle-core

Without sched regression:
this is without sched/fair: Proportional newidle balance

With prefer-idle-core:
https://github.com/marioroy/cachymod/blob/main/linux-cachymod-6.18/
0280-prefer-prevcpu-for-wakeup.patch

Stress-NG 0.20.00: SocketAct, SocketAct24, SocketAct15, SocketAct10
stress-ng -t 30 --metrics-brief --sock -1 --no-rand-seed --sock-zerocopy
stress-ng -t 30 --metrics-brief --sock 24 --no-rand-seed --sock-zerocopy
stress-ng -t 30 --metrics-brief --sock 15 --no-rand-seed --sock-zerocopy
stress-ng -t 30 --metrics-brief --sock 10 --no-rand-seed --sock-zerocopy

Basically 100%, 50%, and 31.25% times 2 (writer, reader)
I ran also, --sock 10 because 10 x 2 is less than 50% (24 threads)

Linux 6.18.7 results: granted, both are built with LTO + AutoFDO profile

CachyOS 6.18.7-2  CachyMod 6.18.7-2 [1]
SocketAct    40799.2           46784.3
SocketAct24  61057.6           71414.5
SocketAct15  45056.4           61772.3
SocketAct10  32691.6           44244.6

[1] https://github.com/marioroy/cachymod
the sched regression reverted (0040 patch)
prefer-idle-core (0280 patch)

On 1/23/26 6:03 AM, Peter Zijlstra wrote:
> On Fri, Jan 23, 2026 at 11:50:46AM +0100, Peter Zijlstra wrote:
>> 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 :/
> Obviously I found it right after sending this. It's a 4x6 config.
> Meaning it needs newidle to balance between those 4 domains.
>
> Pratheek -- are you guys still considering that SIS_NODE thing? That
> worked really well for workstation chips, but there were some issues on
> Epyc or so.
>
>>> #!/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.