On Wed, Sep 17, 2025 at 3:41 PM Nam Cao <nam...@linutronix.de> wrote: > My question is whether the performance of epoll_wait() with zero > timeout is really that important that we have to complicate > things. If epoll_wait() with zero timeout is called repeatedly in a loop > but there is no event, I'm sure there will be measurabled performance > drop. But sane user would just use timeout in that case. > > epoll's data is protected by a lock. Therefore I think the most > straightforward solution is just taking the lock before reading the > data. >
I have no idea what the original use case is. I see the author of the patch is cc'ed, so hopefully they will answer. > Lockless is hard to get right and may cause hard-to-debug problems. So > unless this performance drop somehow bothers someone, I would prefer > "keep it simple, stupid". > Well epoll is known to suffer from lock contention, so I would like to think the lockless games were motivated by a real-world need, but I'm not going peruse the history to find out. I can agree the current state concerning ep_events_available() is avoidably error prone and something(tm) should be done. fwiw the refcount thing is almost free on amd64, I have no idea how this pans out on arm64.
