[PATCHSET v3 0/7] io_uring epoll wait support

[Jens Axboe <axboe@kernel.dk>]

Hi,

One issue people consistently run into when converting legacy epoll
event loops with io_uring is that parts of the event loop still needs to
use epoll. And since event loops generally need to wait in one spot,
they add the io_uring fd to the epoll set and continue to use
epoll_wait(2) to wait on events. This is suboptimal on the io_uring
front as there's now an active poller on the ring, and it's suboptimal
as it doesn't give the application the batch waiting (with fine grained
timeouts) that io_uring provides.

This patchset adds support for IORING_OP_EPOLL_WAIT, which does an async
epoll_wait() operation. No sleeping or thread offload is involved, it
relies on the wait_queue_entry callback for retries. With that, then
the above event loops can continue to use epoll for certain parts, but
bundle it all under waiting on the ring itself rather than add the ring
fd to the epoll set.

Patches 1..2 are just prep patches, and patch 3 adds the epoll change
to allow io_uring to queue a callback, if no events are available.
Patches 5..6 are just prep patches on the io_uring side, and patch 7
finally adds IORING_OP_EPOLL_WAIT support

Patches can also be found here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-epoll-wait

and are against 6.14-rc1 + already pending io_uring patches.

Since v2:
- Drop multishot support, to keep the initial version much simpler
- Drop provided buffers support, not required without multishot
- Cleanup epoll bits, notably adding a separate helper for queueing and
  checking for events
- Various other fixes and cleanups

-- 
Jens Axboe