linux-kernel - [PATCH v5 3/4] io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD

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Date:   Thu,  3 Feb 2022 23:34:38 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     io-uring@...r.kernel.org, axboe@...nel.dk, asml.silence@...il.com,
        linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com, Usama Arif <usama.arif@...edance.com>
Subject: [PATCH v5 3/4] io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD_ASYNC

This is done using the RCU data structure (io_ev_fd). eventfd_async
is moved from io_ring_ctx to io_ev_fd which is RCU protected hence
avoiding ring quiesce which is much more expensive than an RCU lock.
io_should_trigger_evfd is already under rcu_read_lock so there is
no extra RCU read-side critical section needed.

Signed-off-by: Usama Arif <usama.arif@...edance.com>
---
 fs/io_uring.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 51602bddb9a8..5ae51ea12f0f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -329,6 +329,7 @@ struct io_submit_state {
 struct io_ev_fd {
 	struct eventfd_ctx	*cq_ev_fd;
 	struct io_ring_ctx	*ctx;
+	unsigned int		eventfd_async: 1;
 	struct rcu_head		rcu;
 	bool 			unregistering;
 };
@@ -342,7 +343,6 @@ struct io_ring_ctx {
 		unsigned int		flags;
 		unsigned int		compat: 1;
 		unsigned int		drain_next: 1;
-		unsigned int		eventfd_async: 1;
 		unsigned int		restricted: 1;
 		unsigned int		off_timeout_used: 1;
 		unsigned int		drain_active: 1;
@@ -1756,7 +1756,7 @@ static void io_eventfd_signal(struct io_ring_ctx *ctx)
 	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
 		goto out;
 
-	if (!ctx->eventfd_async || io_wq_current_is_worker())
+	if (!ev_fd->eventfd_async || io_wq_current_is_worker())
 		eventfd_signal(ev_fd->cq_ev_fd, 1);
 
 out:
@@ -9377,7 +9377,8 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
 	return done ? done : err;
 }
 
-static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
+static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
+			       unsigned int eventfd_async)
 {
 	struct io_ev_fd *ev_fd;
 	__s32 __user *fds = arg;
@@ -9416,6 +9417,7 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
 	}
 	ev_fd->ctx = ctx;
 	ev_fd->unregistering = false;
+	ev_fd->eventfd_async = eventfd_async;
 
 	rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
 	ret = 0;
@@ -11029,6 +11031,7 @@ static bool io_register_op_must_quiesce(int op)
 	case IORING_UNREGISTER_FILES:
 	case IORING_REGISTER_FILES_UPDATE:
 	case IORING_REGISTER_EVENTFD:
+	case IORING_REGISTER_EVENTFD_ASYNC:
 	case IORING_UNREGISTER_EVENTFD:
 	case IORING_REGISTER_PROBE:
 	case IORING_REGISTER_PERSONALITY:
@@ -11129,17 +11132,16 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 		ret = io_register_files_update(ctx, arg, nr_args);
 		break;
 	case IORING_REGISTER_EVENTFD:
-	case IORING_REGISTER_EVENTFD_ASYNC:
 		ret = -EINVAL;
 		if (nr_args != 1)
 			break;
-		ret = io_eventfd_register(ctx, arg);
-		if (ret)
+		ret = io_eventfd_register(ctx, arg, 0);
+		break;
+	case IORING_REGISTER_EVENTFD_ASYNC:
+		ret = -EINVAL;
+		if (nr_args != 1)
 			break;
-		if (opcode == IORING_REGISTER_EVENTFD_ASYNC)
-			ctx->eventfd_async = 1;
-		else
-			ctx->eventfd_async = 0;
+		ret = io_eventfd_register(ctx, arg, 1);
 		break;
 	case IORING_UNREGISTER_EVENTFD:
 		ret = -EINVAL;
-- 
2.25.1