[PATCHSET for-next 0/3] Add FMODE_NOWAIT support to pipes

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One thing that's always been a bit slower than I'd like with io_uring is
dealing with pipes. They don't support IOCB_NOWAIT, and hence we need to
punt them to io-wq for handling.

This series adds support for FMODE_NOWAIT to pipes.

Patch 1 extends pipe_buf_operations->confirm() to accept a nonblock
parameter, and wires up the caller, pipe_buf_confirm(), to have that
argument too.

Patch 2 makes pipes deal with IOCB_NOWAIT for locking the pipe, calling
pipe_buf_confirm(), and for allocating new pages on writes.

Patch 3 flicks the switch and enables FMODE_NOWAIT for pipes.

Curious on how big of a difference this makes, I wrote a small benchmark
that simply opens 128 pipes and then does 256 rounds of reading and
writing to them. This was run 10 times, discarding the first run as it's
always a bit slower. Before the patch:

Avg:	262.52 msec
Stdev:	  2.12 msec
Min:	261.07 msec
Max	267.91 msec

and after the patch:

Avg:	24.14 msec
Stdev:	 9.61 msec
Min:	17.84 msec
Max:	43.75 msec

or about a 10x improvement in performance (and efficiency).

I ran the patches through the ltp pipe and splice tests, no regressions
observed. Looking at io_uring traces, we can see that we no longer have
any io_uring_queue_async_work() traces after the patch, where previously
everything was done via io-wq.

Jens Axboe

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