[PATCHSET RFC 0/3] Add support for registered waits

[Jens Axboe <axboe@kernel.dk>]

Hi,

While doing testing on zero-copy tx/rx with io_uring, I noticed that a
LOT of time is being spent copying in data structures related to waiting
on events. While the structures aren't big (24b and 16b), it's a user
copy, and a dependent user copy on top of that - first get the main
struct io_uring_getevents_arg, and then copy in the timeout from that.
Details are in patch 3 on the numbers.

I then toyed around with the idea of having registered regions of wait
data. This can be set by the application, and then directly seen by the
kernel. On top of that, embed the timeout itself in this region, rather
than it being supplied as an out-of-band pointer.

Patch 1 is just a generic cleanup, and patch 2 improves the copying a
both. Both of these can stand alone. Patch 3 implements the meat of this,
which adds IORING_REGISTER_CQWAIT_REG, allowing an application to
register a number of struct io_uring_reg_wait regions that it can index
for wait scenarios. The kernel always registers a full page, so on 4k
page size archs, 64 regions are available by default. Then rather than
pass in a pointer to a struct io_uring_getevents_arg, an index is passed
instead, telling the kernel which registered wait region should be used
for this wait.

This basically removes all of the copying seen, which was anywhere from
3.5-4.5% of the time, when doing high frequency operations where
the number of wait invocations can be quite high.

Patches sit on top of the io_uring-ring-resize branch, as both end up
adding register opcodes.

Kernel branch here:

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

and liburing (pretty basic right now) branch here:

https://git.kernel.dk/cgit/liburing/log/?h=reg-wait

 include/linux/io_uring_types.h |   7 +++
 include/uapi/linux/io_uring.h  |  18 ++++++
 io_uring/io_uring.c            | 102 ++++++++++++++++++++++++++-------
 io_uring/register.c            |  48 ++++++++++++++++
 4 files changed, 153 insertions(+), 22 deletions(-)

-- 
Jens Axboe