Re: [PATCH v2 0/2] Optimise io_uring completion waiting

[Jens Axboe <axboe@kernel.dk>]

On 9/24/19 5:23 AM, Pavel Begunkov wrote:
>> Yep that should do it, and saves 8 bytes of stack as well.
>>
>> BTW, did you test my patch, this one or the previous? Just curious if it
>> worked for you.
>>
> Not yet, going to do that tonight

Thanks! For reference, the final version is below. There was still a
signal mishap in there, now it should all be correct afaict.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9b84232e5cc4..d2a86164d520 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2768,6 +2768,38 @@ 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;
+	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))
+		return -1;
+
+	return autoremove_wake_function(curr, mode, wake_flags, key);
+}
+
 /*
  * 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 +2807,16 @@ 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 = {
+			.private	= current,
+			.func		= io_wake_function,
+			.entry		= LIST_HEAD_INIT(iowq.wq.entry),
+		},
+		.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 +2835,20 @@ 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);
+	do {
+		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
+						TASK_INTERRUPTIBLE);
+		if (io_should_wake(&iowq))
+			break;
+		schedule();
+		if (signal_pending(current)) {
+			ret = -ERESTARTSYS;
+			break;
+		}
+	} while (1);
+	finish_wait(&ctx->wait, &iowq.wq);
+
 	restore_saved_sigmask_unless(ret == -ERESTARTSYS);
 	if (ret == -ERESTARTSYS)
 		ret = -EINTR;

-- 
Jens Axboe