linux-kernel - [REGRESSION][v6.17-rc1]sched/fair: Bump sd->max_newidle_lb

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Message-ID: <006c9df2-b691-47f1-82e6-e233c3f91faf@oracle.com>
Date: Mon, 6 Oct 2025 16:23:19 -0400
From: Joseph Salisbury <joseph.salisbury@...cle.com>
To: clm@...com
Cc: Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: [REGRESSION][v6.17-rc1]sched/fair: Bump sd->max_newidle_lb_cost when
 newidle balance fails

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?

Thanks,

Joe


[0]https://openbenchmarking.org/test/pts/apache-3.0.0