Re: [REGRESSION] osnoise: "eventpoll: Replace rwlock with spinlock" causes ~50us noise spikes on isolated PREEMPT_RT cores

Jan Kiszka
2026-03-28T06:00:21Z

On 27.03.26 19:36, Ionut Nechita (Wind River) wrote:
> From: Ionut Nechita <ionut.nechita@windriver.com>
>
> On Thu, 2026-03-27 at 08:44 +0100, Florian Bezdeka wrote:
>> A revert alone is not an option as it would bring back [1] and [2]
>> for all LTS releases that did not receive [3].
>
> Florian, Crystal, thanks for the feedback.
>
> I understand the revert concern regarding the CFS throttle deadlock.
> However, I want to clarify that the noise regression on isolated cores
> is a separate issue from the deadlock fixed by [3], and it remains
> unfixed even on linux-next which has [3] merged or not.
>
> I've done extensive testing across multiple kernels to identify the
> exact mechanism. Here are the results.
>
> Tool: eBPF-based osnoise tracer (https://gitlab.com/rt-linux-tools/eosnoise)
> which uses perf_event_open() + epoll on each monitored CPU, combined
> with /proc/interrupts delta measurement.
>
> Setup:
> - Hardware: x86_64, SMT/HT enabled (CPUs 0-63)

I think Crystal already asked: Are you disabling HT then by taking the
the siblings offline for the isolated cores? If not, the measurements
are a bit questionable from an RT perspective.

Jan

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