linux-kernel - Re: [PATCH v12 9/9] timers: Exclude isolated cpus from timer migration

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Message-ID: <aMh8Oq6El_xV9Ls4@thinkpad2024>
Date: Mon, 15 Sep 2025 16:51:06 -0400
From: "John B. Wyatt IV" <jwyatt@...hat.com>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	cgroups@...r.kernel.org,
	"John B. Wyatt IV" <sageofredondo@...il.com>
Subject: Re: [PATCH v12 9/9] timers: Exclude isolated cpus from timer
 migration

On Mon, Sep 15, 2025 at 04:59:30PM +0200, Gabriele Monaco wrote:

Your patchset continues to pass when applied against v6.17-rc4-rt3 on a
preview of RHEL 10.2.

rtla osnoise top -c 1 -e sched:sched_switch -s 20 -T 1 -t -d 30m -q

duration:   0 00:30:00 | time is in us
CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI          IRQ      Softirq       Thread
  1 #1799     1799000001      3351316    99.81371        2336            9         400            0      1799011            0        23795

> This effect was noticed on a 128 cores machine running oslat on the
> isolated cores (1-31,33-63,65-95,97-127). The tool monopolises CPUs,
> and the CPU with lowest count in a timer migration hierarchy (here 1
> and 65) appears as always active and continuously pulls global timers,
> from the housekeeping CPUs. This ends up moving driver work (e.g.
> delayed work) to isolated CPUs and causes latency spikes:
> 

If you do another version; you may want to amend the cover letter to include
this affect can be noticed with a machine with as few as 20cores/40threads
with isocpus set to: 1-9,11-39 with rtla-osnoise-top

Tested-by: John B. Wyatt IV <jwyatt@...hat.com>
Tested-by: John B. Wyatt IV <sageofredondo@...il.com>

-- 
Sincerely,
John Wyatt
Software Engineer, Core Kernel
Red Hat