Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller

Sebastian Andrzej Siewior
2025-04-09T13:21:52Z

On 2025-04-09 08:41:44 [+0200], Jan Kiszka wrote:
> We are hunting for quite some time sporadic lock-ups or RT systems,
> first only in the field (sigh), now finally also in the lab. Those have
> a fairly high overlap with what was described here. Our baselines so
> far: 6.1-rt, Debian and vanilla. We are currently preparing experiments
> with latest mainline.
>
> While this thread remained silent afterwards, we have found [1][2][3] as
> apparently related. But this means we are still with this RT bug, even
> in latest 6.15-rc1?

Not sure the commits are related. The problem here is that RW locks are
not really real time friendly. Frederick had a simple fix to it
https://lore.kernel.org/all/20210825132754.GA895675@lothringen/

but yeah. The alternative, which I didn't look into, would be to replace
the reader side with RCU so we would just have the writer lock. That
mean we need to RW lock because of performance…

> Jan
>
> [1] https://lore.kernel.org/lkml/20231030145104.4107573-1-vschneid@redhat.com/
> [2] https://lore.kernel.org/lkml/20240202080920.3337862-1-vschneid@redhat.com/
> [3] https://lore.kernel.org/lkml/20250220093257.9380-1-kprateek.nayak@amd.com/
>

Sebastian