On 5/24/24 23:58, Jens Axboe wrote:
If IORING_SETUP_SINGLE_ISSUER 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@xxxxxxxxx>
---
io_uring/msg_ring.c | 77 +++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 74 insertions(+), 3 deletions(-)
diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index feff2b0822cf..3f89ff3a40ad 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -123,6 +123,69 @@ static void io_msg_tw_complete(struct callback_head *head)
io_req_queue_tw_complete(req, ret);
}
+static struct io_overflow_cqe *io_alloc_overflow(struct io_ring_ctx *target_ctx)
+{
+ 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)
+ __releases(target_ctx->uring_lock)
+{
+ spin_lock(&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);
TASKRUN? The normal overflow path sets IORING_SQ_CQ_OVERFLOW
+ }
+
+ ocqe->cqe.user_data = msg->user_data;
+ ocqe->cqe.res = ret;
+ list_add_tail(&ocqe->list, &target_ctx->cq_overflow_list);
+ spin_unlock(&target_ctx->completion_lock);
+ mutex_unlock(&target_ctx->uring_lock);
+ wake_up_state(target_ctx->submitter_task, TASK_INTERRUPTIBLE);
+}
--
Pavel Begunkov
