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

[Mikulas Patocka]

On Thu, 29 Nov 2018, Mike Snitzer wrote:

> On Tue, Nov 27 2018 at  7:42pm -0500,
> Mikulas Patocka <mpatocka at 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 at 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

I know.

It depends on whether Jens takes these patches or not. If he doesn't take 
them, dm will use percpu counters on its own and it will return zero in 
the "/sys/block/dm-*/inflight" file.

Mikulas