Re: [PATCH 2/3] io_uring/msg_ring: avoid double indirection task_work for data messages

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

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@kernel.dk>
> ---
>   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