[PATCH v1] io_uring: Use io_schedule* in cqring wait

[PATCH v1] io_uring: Use io_schedule* in cqring wait

[Andres Freund]

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: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: io-uring@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andres Freund <andres@anarazel.de>
---
 io_uring/io_uring.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 3bca7a79efda..4661a39de716 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2575,6 +2575,9 @@ 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 ret;
+	int token;
+
 	if (unlikely(READ_ONCE(ctx->check_cq)))
 		return 1;
 	if (unlikely(!llist_empty(&ctx->work_llist)))
@@ -2585,11 +2588,20 @@ 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;
 }
 
 /*
-- 
2.38.0

Re: [PATCH v1] io_uring: Use io_schedule* in cqring wait

[Jens Axboe]

On 7/7/23 10:20?AM, Andres Freund wrote:
> 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.

This looks good to me, and I also separately tested a similar patch and
it showed good results for me even with a heavily performance oriented
setup:

	pread2		io_uring	io_uring w/io_sched
QD1	185K		170K		186K
QD2	NA		304K		327K
QD4	NA		630K		640K
QD8	NA		891K		892K

I'll add this, with just one small minor cosmetic edit:

> @@ -2575,6 +2575,9 @@ 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 ret;
> +	int token;

Should just be a single line.

And I'll mark this for stable as well. Thanks!

-- 
Jens Axboe

Re: [PATCH v1] io_uring: Use io_schedule* in cqring wait

[Jens Axboe]

On Fri, 07 Jul 2023 09:20:07 -0700, Andres Freund wrote:
> 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
> 
> [...]

Applied, thanks!

[1/1] io_uring: Use io_schedule* in cqring wait
      (no commit info)

Best regards,
-- 
Jens Axboe