[PATCHSET RFC 0/4] Add support for IOSQE2_POLL_FIRST

[Jens Axboe <axboe@kernel.dk>]

Hi,

For some workloads, it's not at all uncommon that every request will end
up using the internal poll feature to trigger the successful execution of
a request. This is quite common for network receive, where the application
doesn't expect any data to be immediately available. Yet we still attempt
to do this receive, then get -EAGAIN, arm poll, and trigger the retry
based on poll.

This can be quite wasteful, and particularly so for cases where we
expect to arm poll basically 100% of the time.

This series builds to adding support for asking io_uring to arm poll
first, rather than first attempt an IO, and finally adds support for
this feature to send/sendmsg and recv/recvmsg (with the two latter ones
being the most useful, imho).

Given that most requests don't support IO priorities, a new flags2 field
is added using that same space. The last bit we have in sqe->flags is
added to say that "ioprio is really flags2". This does mean that any
IOSQE2_ flags added cannot be used with IO priorities.

-- 
Jens Axboe