Re: [PATCH 5/5] io_uring/epoll: add support for IORING_OP_EPOLL_WAIT

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

On 2/19/25 17:22, Jens Axboe wrote:
> For existing epoll event loops that can't fully convert to io_uring,
> the used approach is usually to add the io_uring fd to the epoll
> instance and use epoll_wait() to wait on both "legacy" and io_uring
> events. While this work, it isn't optimal as:
> 
> 1) epoll_wait() is pretty limited in what it can do. It does not support
>     partial reaping of events, or waiting on a batch of events.
> 
> 2) When an io_uring ring is added to an epoll instance, it activates the
>     io_uring "I'm being polled" logic which slows things down.
> 
> Rather than use this approach, with EPOLL_WAIT support added to io_uring,
> event loops can use the normal io_uring wait logic for everything, as
> long as an epoll wait request has been armed with io_uring.
> 
> Note that IORING_OP_EPOLL_WAIT does NOT take a timeout value, as this
> is an async request. Waiting on io_uring events in general has various
> timeout parameters, and those are the ones that should be used when
> waiting on any kind of request. If events are immediately available for
> reaping, then This opcode will return those immediately. If none are
> available, then it will post an async completion when they become
> available.
> 
> cqe->res will contain either an error code (< 0 value) for a malformed
> request, invalid epoll instance, etc. It will return a positive result
> indicating how many events were reaped.
> 
> IORING_OP_EPOLL_WAIT requests may be canceled using the normal io_uring
> cancelation infrastructure.

Looks quite trivial now! Should be pretty easy to add
multishot on top.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

-- 
Pavel Begunkov