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

[Mike Snitzer]

On Fri, Nov 30 2018 at  4:52pm -0500,
Jens Axboe <axboe at kernel.dk> wrote:

> On 11/30/18 2:44 PM, 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/block/genhd.c
> >> ===================================================================
> >> --- linux-block.orig/block/genhd.c	2018-11-28 00:09:59.000000000 +0100
> >> +++ linux-block/block/genhd.c	2018-11-28 00:09:59.000000000 +0100
> >> @@ -45,53 +45,76 @@ static void disk_add_events(struct gendi
> >>  static void disk_del_events(struct gendisk *disk);
> >>  static void disk_release_events(struct gendisk *disk);
> >>  
> >> -void part_inc_in_flight(struct request_queue *q, struct hd_struct *part, int rw)
> >> +void part_inc_in_flight(struct request_queue *q, int cpu, struct hd_struct *part, int rw)
> >>  {
> >>  	if (queue_is_mq(q))
> >>  		return;
> >>  
> >> -	atomic_inc(&part->in_flight[rw]);
> >> +	local_inc(per_cpu_ptr(part->dkstats, cpu)->in_flight);
> >>  	if (part->partno)
> >> -		atomic_inc(&part_to_disk(part)->part0.in_flight[rw]);
> >> +		local_inc(per_cpu_ptr(part_to_disk(part)->part0.dkstats, cpu)->in_flight);
> >>  }
> >>  
> >> -void part_dec_in_flight(struct request_queue *q, struct hd_struct *part, int rw)
> >> +void part_dec_in_flight(struct request_queue *q, int cpu, struct hd_struct *part, int rw)
> >>  {
> >>  	if (queue_is_mq(q))
> >>  		return;
> >>  
> >> -	atomic_dec(&part->in_flight[rw]);
> >> +	local_dec(per_cpu_ptr(part->dkstats, cpu)->in_flight);
> >>  	if (part->partno)
> >> -		atomic_dec(&part_to_disk(part)->part0.in_flight[rw]);
> >> +		local_dec(per_cpu_ptr(part_to_disk(part)->part0.dkstats, cpu)->in_flight);
> >>  }
> > 
> > 
> > The above is wrong, needs this incremental fix:
> 
> Should also be using this_cpu_inc/dec instead.

How so?  Mikulas is using local_t (original header speaks to why)