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

[Gabriele Monaco <gmonaco@redhat.com>]

2025-09-15T20:51:21Z 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>
>

Thanks John for testing again, I'll mention your results with the next version.

Cheers,
Gabriele