Re: FAILED: patch "[PATCH] io_uring: gate iowait schedule on having pending requests" failed to apply to 5.15-stable tree

[Jens Axboe <axboe@kernel.dk>]

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On 7/31/23 11:53?PM, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.15-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-5.15.y
> git checkout FETCH_HEAD
> git cherry-pick -x 7b72d661f1f2f950ab8c12de7e2bc48bdac8ed69
> # <resolve conflicts, build, test, etc.>
> git commit -s
> git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2023080154-handed-folic-f52f@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Here's one for both 5.10-stable ad 5.15-stable.

-- 
Jens Axboe

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From bca63954fe0022a9c0dc06b94f68271c956875b2 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Tue, 1 Aug 2023 08:39:47 -0600
Subject: [PATCH] io_uring: gate iowait schedule on having pending requests

Commit 7b72d661f1f2f950ab8c12de7e2bc48bdac8ed69 upstream.

A previous commit made all cqring waits marked as iowait, as a way to
improve performance for short schedules with pending IO. However, for
use cases that have a special reaper thread that does nothing but
wait on events on the ring, this causes a cosmetic issue where we
know have one core marked as being "busy" with 100% iowait.

While this isn't a grave issue, it is confusing to users. Rather than
always mark us as being in iowait, gate setting of current->in_iowait
to 1 by whether or not the waiting task has pending requests.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/io-uring/CAMEGJJ2RxopfNQ7GNLhr7X9=bHXKo+G5OOe0LUq=+UgLXsv1Xg@mail.gmail.com/
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217699
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217700
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: Phil Elwell <phil@raspberrypi.com>
Tested-by: Andres Freund <andres@anarazel.de>
Fixes: 8a796565cec3 ("io_uring: Use io_schedule* in cqring wait")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/io_uring.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index d7f87157be9a..70f3377f675c 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -7794,12 +7794,21 @@ static int io_run_task_work_sig(void)
 	return -EINTR;
 }
 
+static bool current_pending_io(void)
+{
+	struct io_uring_task *tctx = current->io_uring;
+
+	if (!tctx)
+		return false;
+	return percpu_counter_read_positive(&tctx->inflight);
+}
+
 /* when returns >0, the caller should retry */
 static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 					  struct io_wait_queue *iowq,
 					  ktime_t *timeout)
 {
-	int token, ret;
+	int io_wait, ret;
 
 	/* make sure we run task_work before checking for signals */
 	ret = io_run_task_work_sig();
@@ -7810,15 +7819,17 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 		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.
+	 * Mark us as being in io_wait if we have pending requests, 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();
+	io_wait = current->in_iowait;
+	if (current_pending_io())
+		current->in_iowait = 1;
 	ret = 1;
 	if (!schedule_hrtimeout(timeout, HRTIMER_MODE_ABS))
 		ret = -ETIME;
-	io_schedule_finish(token);
+	current->in_iowait = io_wait;
 	return ret;
 }
 
-- 
2.40.1