On 09.04.25 15:41, Jan Kiszka wrote: > On 09.04.25 15:21, Sebastian Andrzej Siewior wrote: >> 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… >> > > We know that epoll is not a good idea for RT programs. However, our > problem is that already non-RT programs manage to lock up an RT-enabled > system. On second glance, Frederic's patch would probably also avoid the issue we are seeing as it should bring PI to the CFS-throttled read-lock holder (which is now a write-lock holder). But given how old that proposal is, I assume the performance impact was even for the RT kernel too much, wasn't it? Jan -- Siemens AG, Foundational Technologies Linux Expert Center
