Problems replacing epoll with io_uring in tevent

[Jens Axboe]

On 10/26/22 10:00 AM, Stefan Metzmacher wrote:
> 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.

No problem - have you been able to test the current repo in general? I want to
cut a 2.3 release shortly, but since that particular change impacts any kind of
cqe waiting, would be nice to have a bit more confidence in it.

>>     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.

Yes, this is a bit tricky as we hold the file ref across the operation. I'd
be interested in seeing your approach to this, and also how it would
interact with registered files...

>>     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

Any chance you can do a run with just IORING_SETUP_COOP_TASKRUN set? I'm
curious how big of an impact the IPI elimination is, where it slots in
compared to the defer taskrun and the default settings.

>>        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.

OK

-- 
Jens Axboe