Hi Jens,
9. The above works mostly, but manual testing and our massive automated regression tests found the following problems: a) Related to https://github.com/axboe/liburing/issues/684 I was also wondering about the return value of io_uring_submit_and_wait_timeout(), but in addition I noticed that the timeout parameter doesn't work as expected, the function will wait for two times of the timeout value. I hacked a fix here: https://git.samba.org/?p=metze/samba/wip.git;a=commitdiff;h=06fec644dd9f5748952c8b875878e0e1b0000d33
Thanks for doing an upstream fix for the problem.
b) The major show stopper is that IORING_OP_POLL_ADD calls fget(), while it's pending. Which means that a close() on the related file descriptor is not able to remove the last reference! This is a problem for points 3.d, 4.a and 4.b from above. I doubt IORING_ASYNC_CANCEL_FD would be able to be used as there's not always code being triggered around a raw close() syscall, which could do a sync cancel. For now I plan to epoll_ctl (or IORING_OP_EPOLL_CTL) and only register the fd from epoll_create() with IORING_OP_POLL_ADD or I keep epoll_wait() as blocking call and register the io_uring fd with epoll. I looked at the related epoll code and found that it uses a list in struct file->f_ep to keep the reference, which gets detached also via eventpoll_release_file() called from __fput() Would it be possible move IORING_OP_POLL_ADD to use a similar model so that close() will causes a cqe with -ECANCELED?
I'm currently trying to prototype for an IORING_POLL_CANCEL_ON_CLOSE flag that can be passed to POLL_ADD. With that we'll register the request in &req->file->f_uring_poll (similar to the file->f_ep list for epoll) Then we only get a real reference to the file during the call to vfs_poll() otherwise we drop the fget/fput reference and rely on an io_uring_poll_release_file() (similar to eventpoll_release_file()) to cancel our registered poll request.
c) A simple pipe based performance test shows the following numbers: - 'poll': Got 232387.31 pipe events/sec - 'epoll': Got 251125.25 pipe events/sec - 'samba_io_uring_ev': Got 210998.77 pipe events/sec So the io_uring backend is even slower than the 'poll' backend. I guess the reason is the constant re-submission of IORING_OP_POLL_ADD.
Added some feature autodetection today and I'm now using IORING_SETUP_COOP_TASKRUN, IORING_SETUP_TASKRUN_FLAG, IORING_SETUP_SINGLE_ISSUER and IORING_SETUP_DEFER_TASKRUN if supported by the kernel. On a 6.1 kernel this improved the performance a lot, it's now faster than the epoll backend. The key flag is IORING_SETUP_DEFER_TASKRUN. On a different system than above I'm getting the following numbers: - epoll: Got 114450.16 pipe events/sec - poll: Got 105872.52 pipe events/sec - samba_io_uring_ev-without-defer_taskrun': Got 95564.22 pipe events/sec - samba_io_uring_ev-with-defer_taskrun': Got 122853.85 pipe events/sec
My hope would be that IORING_POLL_ADD_MULTI + IORING_POLL_ADD_LEVEL would be able to avoid the performance problem with samba_io_uring_ev compared to epoll.
I've started with a IORING_POLL_ADD_MULTI + IORING_POLL_ADD_LEVEL prototype, but it's not very far yet and due to the IORING_SETUP_DEFER_TASKRUN speedup, I'll postpone working on IORING_POLL_ADD_LEVEL. metze
