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

["John B. Wyatt IV" <jwyatt@redhat.com>]

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@redhat.com>
Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>

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