Problems replacing epoll with io_uring in tevent

[Stefan Metzmacher]

Hi Jens,

>> any change to get some feedback on these?
>> https://lore.kernel.org/io-uring/60ce8938-77ed-0b43-0852-7895140c3553@samba.org/
>> and
>> https://lore.kernel.org/io-uring/c9a5b180-322c-1eb6-2392-df9370aeb45c@samba.org/
>>
>> Thanks in advance!
> 
> Finally getting around to this after the break...
> 
> I think your initial patch looks reasonable for doing cancel-on-close.
> Can you resubmit it against for-6.3/io_uring so we can get it moving
> forward, hopefully?
> 
> That would also be a good point to discuss the fixed file case as well,
> as ideally this should obviously work on both types.

I rebased on for-6.3/io_uring from over a week ago:
https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/oem-6.X-metze

root at ub1704-167:~/samba.git# uname -a
Linux ub1704-167 6.2.0-rc5-metze.01-00809-g3ffcd1640c8d #1 SMP PREEMPT_DYNAMIC Mon Jan 23 22:56:13 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

I modified the test a bit in order to only test fd events (without signal events),
with this samba code:
https://git.samba.org/?p=metze/samba/wip.git;a=shortlog;h=refs/heads/master-io-uring-native-ops
https://git.samba.org/?p=metze/samba/wip.git;a=shortlog;h=0ddfc6ac8f0bf7c33d0e273b45d9df1951b6452b

It shows that the epoll backend is still the fasted, see below.
I know that the samba_io_uring_ev_native is not really optimized in userspace,
but I fear the limiting factor is the need to re-issue IORING_OP_POLL_ADD
after each completion.

Having IORING_POLL_ADD_MULTI together with a working IORING_POLL_ADD_LEVEL,
might provide something that would be faster than the epoll backend,
but that would mean we would need to maintain a ready list and
issue only a limited amount of completions from the ready list
per io_uring_enter() in order to avoid overflowing the completion queue.
But I'm not sure how to implement that myself...

So IORING_POLL_CANCEL_ON_CLOSE is sadly not enough to be useful for me,
do you think I should submit it anyway even if it's unclear if samba will make use of it
any time soon?

metze

root at ub1704-167:~/samba.git# time bin/smbtorture //a/b local.event.samba_io_uring_ev_native.context
smbtorture 4.19.0pre1-DEVELOPERBUILD
Using seed 1675281295
time: 2023-02-01 19:54:55.272582
test: context
time: 2023-02-01 19:54:55.275092
backend 'samba_io_uring_ev_native' - test_event_context
Got 1000000.00 pipe events
Got 295844.07 pipe events/sec
time: 2023-02-01 19:54:58.655572
success: context

real    0m3,472s
user    0m1,861s
sys     0m1,609s

root at ub1704-167:~/samba.git# time bin/smbtorture //a/b local.event.epoll.context
smbtorture 4.19.0pre1-DEVELOPERBUILD
Using seed 1675281298
time: 2023-02-01 19:54:58.739744
test: context
time: 2023-02-01 19:54:58.741575
backend 'epoll' - test_event_context
Got 1000000.00 pipe events
Got 326759.90 pipe events/sec
time: 2023-02-01 19:55:01.802051
success: context

real    0m3,147s
user    0m1,926s
sys     0m1,218s

root at ub1704-167:~/samba.git# time bin/smbtorture //a/b local.event.poll.context
smbtorture 4.19.0pre1-DEVELOPERBUILD
Using seed 1675281930
time: 2023-02-01 20:05:30.685121
test: context
time: 2023-02-01 20:05:30.686870
backend 'poll' - test_event_context
Got 1000000.00 pipe events
Got 275666.78 pipe events/sec
time: 2023-02-01 20:05:34.314512
success: context

real    0m3,713s
user    0m1,799s
sys     0m1,911s