Commit 8a796565 authored by Andres Freund's avatar Andres Freund Committed by Jens Axboe
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io_uring: Use io_schedule* in cqring wait 



I observed poor performance of io_uring compared to synchronous IO. That
turns out to be caused by deeper CPU idle states entered with io_uring,
due to io_uring using plain schedule(), whereas synchronous IO uses
io_schedule().

The losses due to this are substantial. On my cascade lake workstation,
t/io_uring from the fio repository e.g. yields regressions between 20%
and 40% with the following command:
./t/io_uring -r 5 -X0 -d 1 -s 1 -c 1 -p 0 -S$use_sync -R 0 /mnt/t2/fio/write.0.0

This is repeatable with different filesystems, using raw block devices
and using different block devices.

Use io_schedule_prepare() / io_schedule_finish() in
io_cqring_wait_schedule() to address the difference.

After that using io_uring is on par or surpassing synchronous IO (using
registered files etc makes it reliably win, but arguably is a less fair
comparison).

There are other calls to schedule() in io_uring/, but none immediately
jump out to be similarly situated, so I did not touch them. Similarly,
it's possible that mutex_lock_io() should be used, but it's not clear if
there are cases where that matters.

Cc: stable@vger.kernel.org # 5.10+
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: io-uring@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: default avatarAndres Freund <andres@anarazel.de>
Link: https://lore.kernel.org/r/20230707162007.194068-1-andres@anarazel.de


[axboe: minor style fixup]
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent dfbe5561

io_uring/io_uring.c

+13 −2
Original line number Diff line number Diff line
@@ -2489,6 +2489,8 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx) 
static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,

					  struct io_wait_queue *iowq)

{

	int token, ret;



	if (unlikely(READ_ONCE(ctx->check_cq)))

		return 1;

	if (unlikely(!llist_empty(&ctx->work_llist)))
@@ -2499,11 +2501,20 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, 
		return -EINTR;

	if (unlikely(io_should_wake(iowq)))

		return 0;



	/*

	 * Use io_schedule_prepare/finish, so cpufreq can take into account

	 * that the task is waiting for IO - turns out to be important for low

	 * QD IO.

	 */

	token = io_schedule_prepare();

	ret = 0;

	if (iowq->timeout == KTIME_MAX)

		schedule();

	else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS))

		return -ETIME;

	return 0;

		ret = -ETIME;

	io_schedule_finish(token);

	return ret;

}



/*