Re: [PATCH 3/3] io_uring/msg_ring: improve handling of target CQE posting

[David Wei <dw@davidwei.uk>]

On 2024-03-29 13:09, Jens Axboe wrote:
> Use the exported helper for queueing task_work, rather than rolling our
> own.
> 
> This improves peak performance of message passing by about 5x in some
> basic testing, with 2 threads just sending messages to each other.
> Before this change, it was capped at around 700K/sec, with the change
> it's at over 4M/sec.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  io_uring/msg_ring.c | 24 ++++++------------------
>  1 file changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
> index d1f66a40b4b4..af8a5f2947b7 100644
> --- a/io_uring/msg_ring.c
> +++ b/io_uring/msg_ring.c
> @@ -13,7 +13,6 @@
>  #include "filetable.h"
>  #include "msg_ring.h"
>  
> -
>  /* All valid masks for MSG_RING */
>  #define IORING_MSG_RING_MASK		(IORING_MSG_RING_CQE_SKIP | \
>  					IORING_MSG_RING_FLAGS_PASS)
> @@ -21,7 +20,6 @@
>  struct io_msg {
>  	struct file			*file;
>  	struct file			*src_file;
> -	struct callback_head		tw;
>  	u64 user_data;
>  	u32 len;
>  	u32 cmd;
> @@ -73,26 +71,18 @@ static inline bool io_msg_need_remote(struct io_ring_ctx *target_ctx)
>  	return current != target_ctx->submitter_task;
>  }
>  
> -static int io_msg_exec_remote(struct io_kiocb *req, task_work_func_t func)
> +static int io_msg_exec_remote(struct io_kiocb *req, io_req_tw_func_t func)
>  {
>  	struct io_ring_ctx *ctx = req->file->private_data;
> -	struct io_msg *msg = io_kiocb_to_cmd(req, struct io_msg);
> -	struct task_struct *task = READ_ONCE(ctx->submitter_task);
> -
> -	if (unlikely(!task))
> -		return -EOWNERDEAD;
> -
> -	init_task_work(&msg->tw, func);
> -	if (task_work_add(ctx->submitter_task, &msg->tw, TWA_SIGNAL))
> -		return -EOWNERDEAD;
>  
> +	req->io_task_work.func = func;
> +	io_req_task_work_add_remote(req, ctx, IOU_F_TWQ_LAZY_WAKE);
>  	return IOU_ISSUE_SKIP_COMPLETE;
>  }

This part looks correct. Now with io_req_task_work_add_remote(),
req->io_task_work.func is added to tctx->task_list, and queued up for
execution on the remote ctx->submitter_task via task_work_add(). The end
result is that the argument @func is executed on the remote
ctx->submitter_task, which is the same outcome as before.

Also, unsure how this hand rolled code interacted with defer taskrun
before but now it is handled properly in io_req_task_work_add_remote().

>  
> -static void io_msg_tw_complete(struct callback_head *head)
> +static void io_msg_tw_complete(struct io_kiocb *req, struct io_tw_state *ts)
>  {
> -	struct io_msg *msg = container_of(head, struct io_msg, tw);
> -	struct io_kiocb *req = cmd_to_io_kiocb(msg);
> +	struct io_msg *msg = io_kiocb_to_cmd(req, struct io_msg);
>  	struct io_ring_ctx *target_ctx = req->file->private_data;
>  	int ret = 0;
>  
> @@ -205,10 +195,8 @@ static int io_msg_install_complete(struct io_kiocb *req, unsigned int issue_flag
>  	return ret;
>  }
>  
> -static void io_msg_tw_fd_complete(struct callback_head *head)
> +static void io_msg_tw_fd_complete(struct io_kiocb *req, struct io_tw_state *ts)
>  {
> -	struct io_msg *msg = container_of(head, struct io_msg, tw);
> -	struct io_kiocb *req = cmd_to_io_kiocb(msg);
>  	int ret = -EOWNERDEAD;
>  
>  	if (!(current->flags & PF_EXITING))