Re: [PATCH] io_uring: undeprecate epoll_ctl support

[Ben Noordhuis <info@bnoordhuis.nl>]

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.