On Fri, Nov 30 2018 at 4:52pm -0500,
Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 11/30/18 2:44 PM, Mike Snitzer wrote:
> > On Tue, Nov 27 2018 at 7:42pm -0500,
> > Mikulas Patocka <mpatocka@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>
> >> ---
> >> 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)
