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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        patches@lists.linux.dev, Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andres Freund <andres@anarazel.de>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.15 387/532] io_uring: Use io_schedule* in cqring wait
Date:   Fri, 21 Jul 2023 18:04:51 +0200
Message-ID: <20230721160635.467197216@linuxfoundation.org>
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From: Andres Freund <andres@anarazel.de>

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 io_uring/io_uring.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -7796,7 +7796,7 @@ static inline int io_cqring_wait_schedul
 					  struct io_wait_queue *iowq,
 					  ktime_t *timeout)
 {
-	int ret;
+	int token, ret;
 
 	/* make sure we run task_work before checking for signals */
 	ret = io_run_task_work_sig();
@@ -7806,9 +7806,17 @@ static inline int io_cqring_wait_schedul
 	if (test_bit(0, &ctx->check_cq_overflow))
 		return 1;
 
+	/*
+	 * 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;
 }
 
 /*