Problems replacing epoll with io_uring in tevent

[Stefan Metzmacher]

Hi Jens,

> 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.

At least the timing bug is still fixed (as with my change).

>> 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...

Here's my current patch:
https://git.samba.org/?p=metze/linux/wip.git;a=commitdiff;h=b9cccfac515739fc279c6eec87ce655a96f94685
It compiles, but I haven't tested it yet. And I'm not sure if the locking is done correctly...

>>>      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.

There's no real difference between these:

- no flag
- IORING_SETUP_COOP_TASKRUN|IORING_SETUP_TASKRUN_FLAG
- IORING_SETUP_SINGLE_ISSUER
- IORING_SETUP_COOP_TASKRUN|IORING_SETUP_TASKRUN_FLAG|IORING_SETUP_SINGLE_ISSUER


only these make it fast:
- IORING_SETUP_SINGLE_ISSUER|IORING_SETUP_DEFER_TASKRUN
- IORING_SETUP_COOP_TASKRUN|IORING_SETUP_TASKRUN_FLAG|IORING_SETUP_SINGLE_ISSUER|IORING_SETUP_DEFER_TASKRUN

metze