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

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 3 Mar 2021, JeffleXu wrote:

> 
> 
> 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

No, I submit all the bios and poll for the last one.

> 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.

Could you benchmark it?

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

__split_and_process_bio calls __split_and_process_non_flush in a loop and 
__split_and_process_non_flush records the poll cookie in ci.poll_cookie. 
When we processed all the bios, we poll for the last cookie here:

        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)) ;
        }


Mikulas