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Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
From:   Jens Axboe <axboe@kernel.dk>
To:     Pavel Begunkov <asml.silence@gmail.com>,
        Ingo Molnar <mingo@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
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Date:   Tue, 24 Sep 2019 10:36:28 +0200
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On 9/24/19 2:27 AM, Jens Axboe wrote:
> On 9/24/19 2:02 AM, Jens Axboe wrote:
>> On 9/24/19 1:06 AM, Pavel Begunkov wrote:
>>> On 24/09/2019 02:00, Jens Axboe wrote:
>>>>> I think we can do the same thing, just wrapping the waitqueue in a
>>>>> structure with a count in it, on the stack. Got some flight time
>>>>> coming up later today, let me try and cook up a patch.
>>>>
>>>> Totally untested, and sent out 5 min before departure... But something
>>>> like this.
>>> Hmm, reminds me my first version. Basically that's the same thing but
>>> with macroses inlined. I wanted to make it reusable and self-contained,
>>> though.
>>>
>>> If you don't think it could be useful in other places, sure, we could do
>>> something like that. Is that so?
>>
>> I totally agree it could be useful in other places. Maybe formalized and
>> used with wake_up_nr() instead of adding a new primitive? Haven't looked
>> into that, I may be talking nonsense.
>>
>> In any case, I did get a chance to test it and it works for me. Here's
>> the "finished" version, slightly cleaned up and with a comment added
>> for good measure.
> 
> Notes:
> 
> This version gets the ordering right, you need exclusive waits to get
> fifo ordering on the waitqueue.
> 
> Both versions (yours and mine) suffer from the problem of potentially
> waking too many. I don't think this is a real issue, as generally we
> don't do threaded access to the io_urings. But if you had the following
> tasks wait on the cqring:
> 
> [min_events = 32], [min_events = 8], [min_events = 8]
> 
> and we reach the io_cqring_events() == threshold, we'll wake all three.
> I don't see a good solution to this, so I suspect we just live with
> until proven an issue. Both versions are much better than what we have
> now.

Forgot an issue around signal handling, version below adds the
right check for that too.

Curious what your test case was for this?


diff --git a/fs/io_uring.c b/fs/io_uring.c
index ca7570aca430..3fbab5692f14 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2768,6 +2768,42 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit,
 	return submit;
 }
 
+struct io_wait_queue {
+	struct wait_queue_entry wq;
+	struct io_ring_ctx *ctx;
+	struct task_struct *task;
+	unsigned to_wait;
+	unsigned nr_timeouts;
+};
+
+static inline bool io_should_wake(struct io_wait_queue *iowq)
+{
+	struct io_ring_ctx *ctx = iowq->ctx;
+
+	/*
+	 * Wake up if we have enough events, or if a timeout occured since we
+	 * started waiting. For timeouts, we always want to return to userspace,
+	 * regardless of event count.
+	 */
+	return io_cqring_events(ctx->rings) >= iowq->to_wait ||
+			atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
+}
+
+static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
+			    int wake_flags, void *key)
+{
+	struct io_wait_queue *iowq = container_of(curr, struct io_wait_queue,
+							wq);
+
+	if (io_should_wake(iowq)) {
+		list_del_init(&curr->entry);
+		wake_up_process(iowq->task);
+		return 1;
+	}
+
+	return -1;
+}
+
 /*
  * Wait until events become available, if we don't already have some. The
  * application must reap them itself, as they reside on the shared cq ring.
@@ -2775,8 +2811,16 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit,
 static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 			  const sigset_t __user *sig, size_t sigsz)
 {
+	struct io_wait_queue iowq = {
+		.wq = {
+			.func	= io_wake_function,
+			.entry	= LIST_HEAD_INIT(iowq.wq.entry),
+		},
+		.task		= current,
+		.ctx		= ctx,
+		.to_wait	= min_events,
+	};
 	struct io_rings *rings = ctx->rings;
-	unsigned nr_timeouts;
 	int ret;
 
 	if (io_cqring_events(rings) >= min_events)
@@ -2795,15 +2839,18 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 			return ret;
 	}
 
-	nr_timeouts = atomic_read(&ctx->cq_timeouts);
-	/*
-	 * Return if we have enough events, or if a timeout occured since
-	 * we started waiting. For timeouts, we always want to return to
-	 * userspace.
-	 */
-	ret = wait_event_interruptible(ctx->wait,
-				io_cqring_events(rings) >= min_events ||
-				atomic_read(&ctx->cq_timeouts) != nr_timeouts);
+	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
+	prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
+	do {
+		if (io_should_wake(&iowq))
+			break;
+		schedule();
+		if (signal_pending(current))
+			break;
+		set_current_state(TASK_INTERRUPTIBLE);
+	} while (1);
+	finish_wait(&ctx->wait, &iowq.wq);
+
 	restore_saved_sigmask_unless(ret == -ERESTARTSYS);
 	if (ret == -ERESTARTSYS)
 		ret = -EINTR;

-- 
Jens Axboe