Re: FAILED: patch "[PATCH] io_uring: Use io_schedule* in cqring wait" failed to apply to 6.1-stable tree

[Jens Axboe <axboe@kernel.dk>]

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On 7/16/23 12:13 PM, Jens Axboe wrote:
> On 7/16/23 2:41 AM, gregkh@linuxfoundation.org wrote:
>>
>> The patch below does not apply to the 6.1-stable tree.
>> If someone wants it applied there, or to any other stable or longterm
>> tree, then please email the backport, including the original git commit
>> id to <stable@vger.kernel.org>.
>>
>> To reproduce the conflict and resubmit, you may use the following commands:
>>
>> git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
>> git checkout FETCH_HEAD
>> git cherry-pick -x 8a796565cec3601071cbbd27d6304e202019d014
>> # <resolve conflicts, build, test, etc.>
>> git commit -s
>> git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2023071620-litigate-debunk-939a@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..
> 
> Here's one for 6.1-stable.

And here's a corrected one for 6.1.

-- 
Jens Axboe


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From f5f24ec27340daf12177fd09c2d107a589cbf527 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Sun, 16 Jul 2023 12:13:06 -0600
Subject: [PATCH] io_uring: Use io_schedule* in cqring wait

Commit 8a796565cec3601071cbbd27d6304e202019d014 upstream.

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: Andres Freund <andres@anarazel.de>
Link: https://lore.kernel.org/r/20230707162007.194068-1-andres@anarazel.de
[axboe: minor style fixup]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/io_uring.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index cc35aba1e495..6d7b358e71f1 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2346,7 +2346,7 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 					  struct io_wait_queue *iowq,
 					  ktime_t *timeout)
 {
-	int ret;
+	int token, ret;
 	unsigned long check_cq;
 
 	/* make sure we run task_work before checking for signals */
@@ -2362,9 +2362,18 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 		if (check_cq & BIT(IO_CHECK_CQ_DROPPED_BIT))
 			return -EBADR;
 	}
+
+	/*
+	 * 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 = 1;
 	if (!schedule_hrtimeout(timeout, HRTIMER_MODE_ABS))
-		return -ETIME;
-	return 1;
+		ret = -ETIME;
+	io_schedule_finish(token);
+	return ret;
 }
 
 /*
-- 
2.40.1