[RFC] single cqe per link

[Pavel Begunkov <asml.silence@gmail.com>]

I've got curious about performance of the idea of having only 1 CQE per link
(for the failed or last one). Tested it with a quick dirty patch doing
submit-and-reap of a nops-link (patched for inline execution).

1) link size: 100
old: 206 ns per nop
new: 144 ns per nop

2) link size: 10
old: 234 ns per nop
new: 181 ns per nop

3) link size: 10, FORCE_ASYNC
old: 667 ns per nop
new: 569 ns per nop


The patch below breaks sequences, linked_timeout and who knows what else.
The first one requires synchronisation/atomic, so it's a bit in the way. I've
been wondering, whether IOSQE_IO_DRAIN is popular and how much it's used. We can
try to find tradeoff or even disable it with this feature.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 65a61b8b37c4..9ec29f01cfda 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1164,7 +1164,7 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx
*ctx, bool force)
 	return cqe != NULL;
 }

-static void io_cqring_fill_event(struct io_kiocb *req, long res)
+static void __io_cqring_fill_event(struct io_kiocb *req, long res)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_uring_cqe *cqe;
@@ -1196,13 +1196,31 @@ static void io_cqring_fill_event(struct io_kiocb *req,
long res)
 	}
 }

+static inline bool io_ignore_cqe(struct io_kiocb *req)
+{
+	if (!(req->ctx->flags & IORING_SETUP_BOXED_CQE))
+		return false;
+
+	return (req->flags & (REQ_F_LINK|REQ_F_FAIL_LINK)) == REQ_F_LINK;
+}
+
+static void io_cqring_fill_event(struct io_kiocb *req, long res)
+{
+	if (io_ignore_cqe(req))
+		return;
+	__io_cqring_fill_event(req, res);
+}
+
 static void io_cqring_add_event(struct io_kiocb *req, long res)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned long flags;

+	if (io_ignore_cqe(req))
+		return;
+
 	spin_lock_irqsave(&ctx->completion_lock, flags);
-	io_cqring_fill_event(req, res);
+	__io_cqring_fill_event(req, res);
 	io_commit_cqring(ctx);
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);

@@ -7084,7 +7102,8 @@ static long io_uring_setup(u32 entries, struct
io_uring_params __user *params)

 	if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
 			IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE |
-			IORING_SETUP_CLAMP | IORING_SETUP_ATTACH_WQ))
+			IORING_SETUP_CLAMP | IORING_SETUP_ATTACH_WQ |
+			IORING_SETUP_BOXED_CQE))
 		return -EINVAL;

 	ret = io_uring_create(entries, &p);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 08891cc1c1e7..3d69369e252c 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -86,6 +86,7 @@ enum {
 #define IORING_SETUP_CQSIZE	(1U << 3)	/* app defines CQ size */
 #define IORING_SETUP_CLAMP	(1U << 4)	/* clamp SQ/CQ ring sizes */
 #define IORING_SETUP_ATTACH_WQ	(1U << 5)	/* attach to existing wq */
+#define IORING_SETUP_BOXED_CQE	(1U << 6)	/* single sqe per link */

 enum {
 	IORING_OP_NOP,


-- 
Pavel Begunkov