[PATCH 3/7] io_uring/msg_ring: avoid double indirection task_work for data messages

[Jens Axboe <axboe@kernel.dk>]

If IORING_SETUP_DEFER_TASKRUN is set, then we can't post CQEs remotely
to the target ring. Instead, task_work is queued for the target ring,
which is used to post the CQE. To make matters worse, once the target
CQE has been posted, task_work is then queued with the originator to
fill the completion.

This obviously adds a bunch of overhead and latency. Instead of relying
on generic kernel task_work for this, fill an overflow entry on the
target ring and flag it as such that the target ring will flush it. This
avoids both the task_work for posting the CQE, and it means that the
originator CQE can be filled inline as well.

In local testing, this reduces the latency on the sender side by 5-6x.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/msg_ring.c | 87 ++++++++++++++++++++++++++++-----------------
 1 file changed, 55 insertions(+), 32 deletions(-)

diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index 15e7bda77d0d..bdb935ef7aa2 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -87,38 +87,61 @@ static int io_msg_exec_remote(struct io_kiocb *req, task_work_func_t func)
 	return IOU_ISSUE_SKIP_COMPLETE;
 }
 
-static void io_msg_tw_complete(struct callback_head *head)
+static struct io_overflow_cqe *io_alloc_overflow(struct io_ring_ctx *target_ctx)
 {
-	struct io_msg *msg = container_of(head, struct io_msg, tw);
-	struct io_kiocb *req = cmd_to_io_kiocb(msg);
-	struct io_ring_ctx *target_ctx = req->file->private_data;
-	int ret = 0;
-
-	if (current->flags & PF_EXITING) {
-		ret = -EOWNERDEAD;
-	} else {
-		u32 flags = 0;
-
-		if (msg->flags & IORING_MSG_RING_FLAGS_PASS)
-			flags = msg->cqe_flags;
-
-		/*
-		 * If the target ring is using IOPOLL mode, then we need to be
-		 * holding the uring_lock for posting completions. Other ring
-		 * types rely on the regular completion locking, which is
-		 * handled while posting.
-		 */
-		if (target_ctx->flags & IORING_SETUP_IOPOLL)
-			mutex_lock(&target_ctx->uring_lock);
-		if (!io_post_aux_cqe(target_ctx, msg->user_data, msg->len, flags))
-			ret = -EOVERFLOW;
-		if (target_ctx->flags & IORING_SETUP_IOPOLL)
-			mutex_unlock(&target_ctx->uring_lock);
+	bool is_cqe32 = target_ctx->flags & IORING_SETUP_CQE32;
+	size_t cqe_size = sizeof(struct io_overflow_cqe);
+	struct io_overflow_cqe *ocqe;
+
+	if (is_cqe32)
+		cqe_size += sizeof(struct io_uring_cqe);
+
+	ocqe = kmalloc(cqe_size, GFP_ATOMIC | __GFP_ACCOUNT);
+	if (!ocqe)
+		return NULL;
+
+	if (is_cqe32)
+		ocqe->cqe.big_cqe[0] = ocqe->cqe.big_cqe[1] = 0;
+
+	return ocqe;
+}
+
+/*
+ * Entered with the target uring_lock held, and will drop it before
+ * returning. Adds a previously allocated ocqe to the overflow list on
+ * the target, and marks it appropriately for flushing.
+ */
+static void io_msg_add_overflow(struct io_msg *msg,
+				struct io_ring_ctx *target_ctx,
+				struct io_overflow_cqe *ocqe, int ret,
+				u32 flags)
+	__releases(&target_ctx->completion_lock)
+{
+	if (list_empty(&target_ctx->cq_overflow_list)) {
+		set_bit(IO_CHECK_CQ_OVERFLOW_BIT, &target_ctx->check_cq);
+		atomic_or(IORING_SQ_TASKRUN, &target_ctx->rings->sq_flags);
 	}
 
-	if (ret < 0)
-		req_set_fail(req);
-	io_req_queue_tw_complete(req, ret);
+	ocqe->cqe.user_data = msg->user_data;
+	ocqe->cqe.res = ret;
+	ocqe->cqe.flags = flags;
+	list_add_tail(&ocqe->list, &target_ctx->cq_overflow_list);
+	spin_unlock(&target_ctx->completion_lock);
+	wake_up_state(target_ctx->submitter_task, TASK_INTERRUPTIBLE);
+}
+
+static int io_msg_fill_remote(struct io_msg *msg, unsigned int issue_flags,
+			      struct io_ring_ctx *target_ctx, u32 flags)
+{
+	struct io_overflow_cqe *ocqe;
+
+	ocqe = io_alloc_overflow(target_ctx);
+	if (!ocqe)
+		return -ENOMEM;
+
+	spin_lock(&target_ctx->completion_lock);
+	io_msg_add_overflow(msg, target_ctx, ocqe, msg->len, flags);
+	return 0;
 }
 
 static int io_msg_ring_data(struct io_kiocb *req, unsigned int issue_flags)
@@ -135,12 +158,12 @@ static int io_msg_ring_data(struct io_kiocb *req, unsigned int issue_flags)
 	if (target_ctx->flags & IORING_SETUP_R_DISABLED)
 		return -EBADFD;
 
-	if (io_msg_need_remote(target_ctx))
-		return io_msg_exec_remote(req, io_msg_tw_complete);
-
 	if (msg->flags & IORING_MSG_RING_FLAGS_PASS)
 		flags = msg->cqe_flags;
 
+	if (io_msg_need_remote(target_ctx))
+		return io_msg_fill_remote(msg, issue_flags, target_ctx, flags);
+
 	ret = -EOVERFLOW;
 	if (target_ctx->flags & IORING_SETUP_IOPOLL) {
 		if (unlikely(io_double_lock_ctx(target_ctx, issue_flags)))
-- 
2.43.0