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Date:   Mon, 24 Jul 2023 10:08:39 -0600
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Subject: Re: [PATCH] io_uring: Use io_schedule* in cqring wait
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To:     Phil Elwell <phil@raspberrypi.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>, andres@anarazel.de,
        asml.silence@gmail.com, david@fromorbit.com, hch@lst.de,
        io-uring@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        linux-xfs@vger.kernel.org, stable <stable@vger.kernel.org>
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From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <CAMEGJJ3SjWdJFwzB+sz79ojWqAAMULa2CFAas0tv+JJLJMwoGQ@mail.gmail.com>
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On 7/24/23 10:07?AM, Phil Elwell wrote:
>> Even though I don't think this is an actual problem, it is a bit
>> confusing that you get 100% iowait while waiting without having IO
>> pending. So I do think the suggested patch is probably worthwhile
>> pursuing. I'll post it and hopefully have Andres test it too, if he's
>> available.
> 
> If you CC me I'll happily test it for you.

Here it is.

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 89a611541bc4..f4591b912ea8 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2493,11 +2493,20 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx)
 	return 0;
 }
 
+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)
 {
-	int token, ret;
+	int io_wait, ret;
 
 	if (unlikely(READ_ONCE(ctx->check_cq)))
 		return 1;
@@ -2511,17 +2520,19 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 		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.
+	 * 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 = 0;
 	if (iowq->timeout == KTIME_MAX)
 		schedule();
 	else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS))
 		ret = -ETIME;
-	io_schedule_finish(token);
+	current->in_iowait = io_wait;
 	return ret;
 }
 

-- 
Jens Axboe