Re: [PATCH 2/3] block: switch to per-cpu in-flight counters

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Nov 27 2018 at  7:42pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> Now when part_round_stats is gone, we can switch to per-cpu in-flight
> counters.
> 
> We use the local-atomic type local_t, so that if part_inc_in_flight or
> part_dec_in_flight is reentrantly called from an interrupt, the value will
> be correct.
> 
> The other counters could be corrupted due to reentrant interrupt, but the
> corruption only results in slight counter skew - the in_flight counter
> must be exact, so it needs local_t.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  block/bio.c           |    4 ++--
>  block/blk-core.c      |    4 ++--
>  block/blk-merge.c     |    2 +-
>  block/genhd.c         |   47 +++++++++++++++++++++++++++++++++++------------
>  drivers/md/dm.c       |    4 +---
>  include/linux/genhd.h |    7 ++++---
>  6 files changed, 45 insertions(+), 23 deletions(-)
> 
...
> Index: linux-block/drivers/md/dm.c
> ===================================================================
> --- linux-block.orig/drivers/md/dm.c	2018-11-28 00:09:59.000000000 +0100
> +++ linux-block/drivers/md/dm.c	2018-11-28 00:09:59.000000000 +0100
> @@ -663,8 +663,7 @@ static void start_io_acct(struct dm_io *
>  	generic_start_io_acct(md->queue, bio_op(bio), bio_sectors(bio),
>  			      &dm_disk(md)->part0);
>  
> -	atomic_set(&dm_disk(md)->part0.in_flight[rw],
> -		   atomic_inc_return(&md->pending[rw]));
> +	atomic_inc(&md->pending[rw]);
>  
>  	if (unlikely(dm_stats_used(&md->stats)))
>  		dm_stats_account_io(&md->stats, bio_data_dir(bio),
> @@ -693,7 +692,6 @@ static void end_io_acct(struct dm_io *io
>  	 * a flush.
>  	 */
>  	pending = atomic_dec_return(&md->pending[rw]);
> -	atomic_set(&dm_disk(md)->part0.in_flight[rw], pending);
>  	pending += atomic_read(&md->pending[rw^0x1]);
>  
>  	/* nudge anyone waiting on suspend queue */
> 


These dm.c hunks conflict with changes from you that I already staged in
dm-4.21, see:

https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=b5616f7a11592cc74860f4ec3e3c4fba6688eefa
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=78f95b53c203c969bbe6b86e405f7a891a43b6be

I'd really like to get away from DM maintaining its own ->pending
counters.

Mike