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

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Nov 29 2018 at  5:05pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 29 Nov 2018, Mike Snitzer wrote:
> 
> > 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
> 
> 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.

I see, yeah, I could easily rebase the changes I referenced.

I had a quick look at your proposed changes, they seem pretty clean to
me.  The fact that disk_stats is already percpu for other counters makes
this change straight forward.

But I'll review closer.

Mike