Re: [PATCH] ublk: eliminate unnecessary io_cmds queue

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 09, 2024 at 01:37:00PM -0600, Uday Shankar wrote:
> Currently, ublk_drv maintains a per-hctx queue of requests awaiting
> dispatch to the ublk server, and pokes the ubq_daemon to come pick them
> up via the task_work mechanism when needed. But task_work already
> supports internal (lockless) queueing. Reuse this queueing mechanism
> (i.e. have one task_work queue item per request awaiting dispatch)
> instead of maintaining our own queue in ublk_drv.
> 
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> ---
>  drivers/block/ublk_drv.c | 34 ++++++----------------------------
>  1 file changed, 6 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 60f6d86ea1e6..2ea108347ec4 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -80,6 +80,7 @@ struct ublk_rq_data {
>  
>  struct ublk_uring_cmd_pdu {
>  	struct ublk_queue *ubq;
> +	struct request *req;
>  	u16 tag;
>  };

I'd suggest to add the following build check in init function since there is
only 32bytes in uring_cmd pdu:

	BUILD_BUG_ON(sizeof(ublk_uring_cmd_pdu) < sizeof_field(io_uring_cmd, pud))

>  
> @@ -141,8 +142,6 @@ struct ublk_queue {
>  	struct task_struct	*ubq_daemon;
>  	char *io_cmd_buf;
>  
> -	struct llist_head	io_cmds;
> -
>  	unsigned long io_addr;	/* mapped vm address */
>  	unsigned int max_io_sz;
>  	bool force_abort;
> @@ -1132,9 +1131,10 @@ static inline void __ublk_abort_rq(struct ublk_queue *ubq,
>  		blk_mq_end_request(rq, BLK_STS_IOERR);
>  }
>  
> -static inline void __ublk_rq_task_work(struct request *req,
> +static inline void __ublk_rq_task_work(struct io_uring_cmd *cmd,
>  				       unsigned issue_flags)

`inline` can be removed and __ublk_rq_task_work() can be named as
ublk_rq_task_work() now.

>  {
> +	struct request *req = ublk_get_uring_cmd_pdu(cmd)->req;
>  	struct ublk_queue *ubq = req->mq_hctx->driver_data;
>  	int tag = req->tag;
>  	struct ublk_io *io = &ubq->ios[tag];
> @@ -1211,34 +1211,12 @@ static inline void __ublk_rq_task_work(struct request *req,
>  	ubq_complete_io_cmd(io, UBLK_IO_RES_OK, issue_flags);
>  }
>  
> -static inline void ublk_forward_io_cmds(struct ublk_queue *ubq,
> -					unsigned issue_flags)
> -{
> -	struct llist_node *io_cmds = llist_del_all(&ubq->io_cmds);
> -	struct ublk_rq_data *data, *tmp;
> -
> -	io_cmds = llist_reverse_order(io_cmds);
> -	llist_for_each_entry_safe(data, tmp, io_cmds, node)
> -		__ublk_rq_task_work(blk_mq_rq_from_pdu(data), issue_flags);
> -}
> -
> -static void ublk_rq_task_work_cb(struct io_uring_cmd *cmd, unsigned issue_flags)
> -{
> -	struct ublk_uring_cmd_pdu *pdu = ublk_get_uring_cmd_pdu(cmd);
> -	struct ublk_queue *ubq = pdu->ubq;
> -
> -	ublk_forward_io_cmds(ubq, issue_flags);
> -}
> -
>  static void ublk_queue_cmd(struct ublk_queue *ubq, struct request *rq)
>  {
> -	struct ublk_rq_data *data = blk_mq_rq_to_pdu(rq);
> -
> -	if (llist_add(&data->node, &ubq->io_cmds)) {
> -		struct ublk_io *io = &ubq->ios[rq->tag];
> +	struct ublk_io *io = &ubq->ios[rq->tag];
>  
> -		io_uring_cmd_complete_in_task(io->cmd, ublk_rq_task_work_cb);
> -	}
> +	ublk_get_uring_cmd_pdu(io->cmd)->req = rq;
> +	io_uring_cmd_complete_in_task(io->cmd, __ublk_rq_task_work);
>  }

I'd suggest to comment that io_uring_cmd_complete_in_task() needs to
maintain io command order.

Otherwise this patch looks fine.


Thanks,
Ming