Re: [dm-devel] [PATCH 4/4] dm: support I/O polling

[JeffleXu <jefflexu@linux.alibaba.com>]

On 3/3/21 3:05 AM, Mikulas Patocka wrote:

> Support I/O polling if submit_bio_noacct_mq_direct returned non-empty
> cookie.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  drivers/md/dm.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> Index: linux-2.6/drivers/md/dm.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
> +++ linux-2.6/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
> @@ -1682,6 +1682,11 @@ static void __split_and_process_bio(stru
>  		}
>  	}
>  
> +	if (ci.poll_cookie != BLK_QC_T_NONE) {
> +		while (atomic_read(&ci.io->io_count) > 1 &&
> +		       blk_poll(ci.poll_queue, ci.poll_cookie, true)) ;
> +	}
> +
>  	/* drop the extra reference count */
>  	dec_pending(ci.io, errno_to_blk_status(error));
>  }

It seems that the general idea of your design is to
1) submit *one* split bio
2) blk_poll(), waiting the previously submitted split bio complets

and then submit next split bio, repeating the above process. I'm afraid
the performance may be an issue here, since the batch every time
blk_poll() reaps may decrease.


Besides, the submitting routine and polling routine is bound together
here, i.e., polling is always synchronous.

-- 
Thanks,
Jeffle