| Message ID | 20190119180506.1300-3-snitzer@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | dm: fix various issues with bio splitting code | expand |
On Sat, Jan 19, 2019 at 01:05:04PM -0500, Mike Snitzer wrote: > The risk of redundant IO accounting was not taken into consideration > when commit 18a25da84354 ("dm: ensure bio submission follows a > depth-first tree walk") introduced IO splitting in terms of recursion > via generic_make_request(). > > Fix this by subtracting the split bio's payload from the IO stats that > were already accounted for by start_io_acct() upon dm_make_request() > entry. This repeat oscillation of the IO accounting, up then down, > isn't ideal but refactoring DM core's IO splitting to pre-split bios > _before_ they are accounted turned out to be an excessive amount of > change that will need a full development cycle to refine and verify. > > Before this fix: > > /dev/mapper/stripe_dev is a 4-way stripe using a 32k chunksize, so > bios are split on 32k boundaries. > > # fio --name=16M --filename=/dev/mapper/stripe_dev --rw=write --bs=64k --size=16M \ > --iodepth=1 --ioengine=libaio --direct=1 --refill_buffers > > with debugging added: > [103898.310264] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=0 len=128 > [103898.318704] device-mapper: core: __split_and_process_bio: recursing for following split bio: > [103898.329136] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=64 len=64 > ... > > 16M written yet 136M (278528 * 512b) accounted: > # cat /sys/block/dm-2/stat | awk '{ print $7 }' > 278528 > > After this fix: > > 16M written and 16M (32768 * 512b) accounted: > # cat /sys/block/dm-2/stat | awk '{ print $7 }' > 32768 > > Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk") > Cc: stable@vger.kernel.org # 4.16+ > Reported-by: Bryan Gurney <bgurney@redhat.com> > Signed-off-by: Mike Snitzer <snitzer@redhat.com> > --- > drivers/md/dm.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c > index fcb97b0a5743..fbadda68e23b 100644 > --- a/drivers/md/dm.c > +++ b/drivers/md/dm.c > @@ -1584,6 +1584,9 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md, > ci->sector = bio->bi_iter.bi_sector; > } > > +#define __dm_part_stat_sub(part, field, subnd) \ > + (part_stat_get(part, field) -= (subnd)) > + > /* > * Entry point to split a bio into clones and submit them to the targets. > */ > @@ -1638,6 +1641,19 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md, > struct bio *b = bio_split(bio, bio_sectors(bio) - ci.sector_count, > GFP_NOIO, &md->queue->bio_split); > ci.io->orig_bio = b; > + > + /* > + * Adjust IO stats for each split, otherwise upon queue > + * reentry there will be redundant IO accounting. > + * NOTE: this is a stop-gap fix, a proper fix involves > + * significant refactoring of DM core's bio splitting > + * (by eliminating DM's splitting and just using bio_split) > + */ > + part_stat_lock(); > + __dm_part_stat_sub(&dm_disk(md)->part0, > + sectors[op_stat_group(bio_op(bio))], ci.sector_count); > + part_stat_unlock(); > + > bio_chain(b, bio); > ret = generic_make_request(bio); > break; This ways is a bit ugly, but looks it works and it is simple, especially DM target may accept partial bio, so: Reviewed-by: Ming Lei <ming.lei@redhat.com> Thanks, Ming
diff --git a/drivers/md/dm.c b/drivers/md/dm.c index fcb97b0a5743..fbadda68e23b 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -1584,6 +1584,9 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md, ci->sector = bio->bi_iter.bi_sector; } +#define __dm_part_stat_sub(part, field, subnd) \ + (part_stat_get(part, field) -= (subnd)) + /* * Entry point to split a bio into clones and submit them to the targets. */ @@ -1638,6 +1641,19 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md, struct bio *b = bio_split(bio, bio_sectors(bio) - ci.sector_count, GFP_NOIO, &md->queue->bio_split); ci.io->orig_bio = b; + + /* + * Adjust IO stats for each split, otherwise upon queue + * reentry there will be redundant IO accounting. + * NOTE: this is a stop-gap fix, a proper fix involves + * significant refactoring of DM core's bio splitting + * (by eliminating DM's splitting and just using bio_split) + */ + part_stat_lock(); + __dm_part_stat_sub(&dm_disk(md)->part0, + sectors[op_stat_group(bio_op(bio))], ci.sector_count); + part_stat_unlock(); + bio_chain(b, bio); ret = generic_make_request(bio); break;
The risk of redundant IO accounting was not taken into consideration when commit 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk") introduced IO splitting in terms of recursion via generic_make_request(). Fix this by subtracting the split bio's payload from the IO stats that were already accounted for by start_io_acct() upon dm_make_request() entry. This repeat oscillation of the IO accounting, up then down, isn't ideal but refactoring DM core's IO splitting to pre-split bios _before_ they are accounted turned out to be an excessive amount of change that will need a full development cycle to refine and verify. Before this fix: /dev/mapper/stripe_dev is a 4-way stripe using a 32k chunksize, so bios are split on 32k boundaries. # fio --name=16M --filename=/dev/mapper/stripe_dev --rw=write --bs=64k --size=16M \ --iodepth=1 --ioengine=libaio --direct=1 --refill_buffers with debugging added: [103898.310264] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=0 len=128 [103898.318704] device-mapper: core: __split_and_process_bio: recursing for following split bio: [103898.329136] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=64 len=64 ... 16M written yet 136M (278528 * 512b) accounted: # cat /sys/block/dm-2/stat | awk '{ print $7 }' 278528 After this fix: 16M written and 16M (32768 * 512b) accounted: # cat /sys/block/dm-2/stat | awk '{ print $7 }' 32768 Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk") Cc: stable@vger.kernel.org # 4.16+ Reported-by: Bryan Gurney <bgurney@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> --- drivers/md/dm.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)