Re: [PATCH] io_uring: undeprecate epoll_ctl support

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/3/23 13:49, Jens Axboe wrote:
> On 5/3/23 2:58?AM, Ben Noordhuis wrote:
>> On Tue, May 2, 2023 at 2:51?PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>>
>>> On 5/1/23 19:52, Ben Noordhuis wrote:
>>>> Libuv recently started using it so there is at least one consumer now.
>>>
>>> It was rather deprecated because io_uring controlling epoll is a bad
>>> idea and should never be used. One reason is that it means libuv still
>>> uses epoll but not io_uring, and so the use of io_uring wouldn't seem
>>> to make much sense. You're welcome to prove me wrong on that, why libuv
>>> decided to use a deprecated API in the first place?
>>> Sorry, but the warning is going to stay and libuv should revert the use
>>> of epol_ctl requests.
>>
>> Why use a deprecated API? Because it was only recently deprecated.
>> Distro kernels don't warn about it yet. I only found out because of
>> kernel source code spelunking.
>>
>> Why combine io_uring and epoll? Libuv uses level-triggered I/O for
>> reasons (I can go into detail but they're not material) so it's very
>> profitable to batch epoll_ctl syscalls; it's the epoll_ctlv() syscall
>> people have been asking for since practically forever.
>>
>> Why not switch to io_uring wholesale? Libuv can't drop support for
>> epoll because of old kernels, and io_uring isn't always clearly faster
>> than epoll in the first place.
>>
>> As to the warning: according to the commit that introduced it, it was
>> added because no one was using IORING_OP_EPOLL_CTL. Well, now someone
>> is using it. Saying it's a bad API feels like post-hoc
>> rationalization. I kindly ask you merge this patch. I'd be happy to
>> keep an eye on io_uring/epoll.c if you're worried about maintenance
>> burden.
> 
> This is obviously mostly our fault, as the deprecation patch should've
> obviously been backported to stable. Just adding it to the current
> kernel defeated the purpose, as it added a long period where older
> kernels quite happily accepted epoll use cases.
> 
> So I do agree, the only sane course of action here is to un-deprecate
> it.

nack, keeping piling rubbish is not a great course of action at all.

Has libuv already released it? Because it seems the patches were
just merged.

-- 
Pavel Begunkov