On 19/05/25 11:45, Nam Cao wrote: > On Mon, May 19, 2025 at 11:25:51AM +0200, Florian Bezdeka wrote: >> Hi all, >> >> sorry for top-posting, but I think it makes sense in this case as I'm >> trying to merge different workstreams, likely working on the same >> problem showing up in different colors. >> >> Main goal is to make everybody aware of the other stream / patch >> series. >> >> We have colleagues from Bytedance working on non-RT performance >> optimizations related to CONFIG_CFS_BANDWIDTH at [1]. >> >> This series came to light while searching for a solution for a RT >> lockup, reported at [2]. >> >> We heavily tested [1] during the last month on RT and can report >> success now. In our tests we saw read-lock holder preemption only >> within the epoll interface. It might be that [1] fixes more potential >> issues in this regard. >> >> Today [3] (= the patch I'm replying to, see below) got posted. >> Linutronix reworking the epoll infrastructure. >> >> I would love to learn how/if the combination, basically [1] and [3] fit >> together. > > [1] fixes stall problem involving rw semaphore which epoll uses, but it > doesn't fix the possible priority inversion with epoll > > [3] fixes priority inversion problem with epoll by stop using rw semaphore, > but it doesn't do anything about rw semaphore > > So I propose we keep both. > FWIW I'm thinking the same. [1] fixes the CFS throttling circular dependency on RT but also improves tail latency when CFS throttling is in use (so not just for RT). [3] is specific to epoll but gets rid of the rwsem which is a win overall. I need to go get my reviewing hat back on and stare at both... > Best regards, > Nam > >> My understanding right now is, that [1] fixes a CFS issue, throttling >> while holding a lock is not ideal on !RT - but might cause a critical >> lockup on RT - while [3] is addressing a similar (RT) problem in epoll. >> >> Best regards, >> Florian >> >> [1] https://lore.kernel.org/all/20250409120746.635476-1-ziqianlu@xxxxxxxxxxxxx/ >> [2] https://lore.kernel.org/linux-rt-users/xhsmhttqvnall.mognet@xxxxxxxxxxxxxxxxxxx/ >> [3] https://lore.kernel.org/linux-rt-users/20250519074016.3337326-1-namcao@xxxxxxxxxxxxx/T/#u
