From 4ad8d880b6c4497e365fb6fd16bab52e9974a3f6 Mon Sep 17 00:00:00 2001 From: Nikos Tsironis Date: Sun, 17 Mar 2019 14:22:56 +0200 Subject: [PATCH] dm snapshot: Replace mutex with rw semaphore dm-snapshot uses a single mutex to serialize every access to the snapshot state. This includes all accesses to the complete and pending exception tables, which occur at every origin write, every snapshot read/write and every exception completion. The lock statistics indicate that this mutex is a bottleneck (average wait time ~480 usecs for 8 processes doing random 4K writes to the origin device) preventing dm-snapshot to scale as the number of threads doing IO increases. The major contention points are __origin_write()/snapshot_map() and pending_complete(), i.e., the submission and completion of pending exceptions. Replace this mutex with a rw semaphore. We essentially revert commit ae1093be5a0ef9 ("dm snapshot: use mutex instead of rw_semaphore") and together with the next two patches we substitute the single mutex with a fine-grained locking scheme, where we use a read-write semaphore to protect the mostly read fields of the snapshot structure, e.g., valid, active, etc., and per-bucket bit spinlocks to protect accesses to the complete and pending exception tables. Co-developed-by: Ilias Tsitsimpis Signed-off-by: Nikos Tsironis Acked-by: Mikulas Patocka Signed-off-by: Mike Snitzer --- drivers/md/dm-snap.c | 88 ++++++++++++++++++++++---------------------- 1 file changed, 43 insertions(+), 45 deletions(-) diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c index 051e4d076323..5a67f408876e 100644 --- a/drivers/md/dm-snap.c +++ b/drivers/md/dm-snap.c @@ -48,7 +48,7 @@ struct dm_exception_table { }; struct dm_snapshot { - struct mutex lock; + struct rw_semaphore lock; struct dm_dev *origin; struct dm_dev *cow; @@ -457,9 +457,9 @@ static int __find_snapshots_sharing_cow(struct dm_snapshot *snap, if (!bdev_equal(s->cow->bdev, snap->cow->bdev)) continue; - mutex_lock(&s->lock); + down_read(&s->lock); active = s->active; - mutex_unlock(&s->lock); + up_read(&s->lock); if (active) { if (snap_src) @@ -927,7 +927,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s) int r; chunk_t old_chunk = s->first_merging_chunk + s->num_merging_chunks - 1; - mutex_lock(&s->lock); + down_write(&s->lock); /* * Process chunks (and associated exceptions) in reverse order @@ -942,7 +942,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s) b = __release_queued_bios_after_merge(s); out: - mutex_unlock(&s->lock); + up_write(&s->lock); if (b) flush_bios(b); @@ -1001,9 +1001,9 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s) if (linear_chunks < 0) { DMERR("Read error in exception store: " "shutting down merge"); - mutex_lock(&s->lock); + down_write(&s->lock); s->merge_failed = 1; - mutex_unlock(&s->lock); + up_write(&s->lock); } goto shut; } @@ -1044,10 +1044,10 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s) previous_count = read_pending_exceptions_done_count(); } - mutex_lock(&s->lock); + down_write(&s->lock); s->first_merging_chunk = old_chunk; s->num_merging_chunks = linear_chunks; - mutex_unlock(&s->lock); + up_write(&s->lock); /* Wait until writes to all 'linear_chunks' drain */ for (i = 0; i < linear_chunks; i++) @@ -1089,10 +1089,10 @@ static void merge_callback(int read_err, unsigned long write_err, void *context) return; shut: - mutex_lock(&s->lock); + down_write(&s->lock); s->merge_failed = 1; b = __release_queued_bios_after_merge(s); - mutex_unlock(&s->lock); + up_write(&s->lock); error_bios(b); merge_shutdown(s); @@ -1191,7 +1191,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv) s->exception_start_sequence = 0; s->exception_complete_sequence = 0; s->out_of_order_tree = RB_ROOT; - mutex_init(&s->lock); + init_rwsem(&s->lock); INIT_LIST_HEAD(&s->list); spin_lock_init(&s->pe_lock); s->state_bits = 0; @@ -1357,9 +1357,9 @@ static void snapshot_dtr(struct dm_target *ti) /* Check whether exception handover must be cancelled */ (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL); if (snap_src && snap_dest && (s == snap_src)) { - mutex_lock(&snap_dest->lock); + down_write(&snap_dest->lock); snap_dest->valid = 0; - mutex_unlock(&snap_dest->lock); + up_write(&snap_dest->lock); DMERR("Cancelling snapshot handover."); } up_read(&_origins_lock); @@ -1390,8 +1390,6 @@ static void snapshot_dtr(struct dm_target *ti) dm_exception_store_destroy(s->store); - mutex_destroy(&s->lock); - dm_put_device(ti, s->cow); dm_put_device(ti, s->origin); @@ -1479,7 +1477,7 @@ static void pending_complete(void *context, int success) if (!success) { /* Read/write error - snapshot is unusable */ - mutex_lock(&s->lock); + down_write(&s->lock); __invalidate_snapshot(s, -EIO); error = 1; goto out; @@ -1487,14 +1485,14 @@ static void pending_complete(void *context, int success) e = alloc_completed_exception(GFP_NOIO); if (!e) { - mutex_lock(&s->lock); + down_write(&s->lock); __invalidate_snapshot(s, -ENOMEM); error = 1; goto out; } *e = pe->e; - mutex_lock(&s->lock); + down_write(&s->lock); if (!s->valid) { free_completed_exception(e); error = 1; @@ -1512,9 +1510,9 @@ static void pending_complete(void *context, int success) /* Wait for conflicting reads to drain */ if (__chunk_is_tracked(s, pe->e.old_chunk)) { - mutex_unlock(&s->lock); + up_write(&s->lock); __check_for_conflicting_io(s, pe->e.old_chunk); - mutex_lock(&s->lock); + down_write(&s->lock); } out: @@ -1527,7 +1525,7 @@ out: full_bio->bi_end_io = pe->full_bio_end_io; increment_pending_exceptions_done_count(); - mutex_unlock(&s->lock); + up_write(&s->lock); /* Submit any pending write bios */ if (error) { @@ -1750,7 +1748,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) if (!s->valid) return DM_MAPIO_KILL; - mutex_lock(&s->lock); + down_write(&s->lock); if (!s->valid || (unlikely(s->snapshot_overflowed) && bio_data_dir(bio) == WRITE)) { @@ -1773,9 +1771,9 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) if (bio_data_dir(bio) == WRITE) { pe = __lookup_pending_exception(s, chunk); if (!pe) { - mutex_unlock(&s->lock); + up_write(&s->lock); pe = alloc_pending_exception(s); - mutex_lock(&s->lock); + down_write(&s->lock); if (!s->valid || s->snapshot_overflowed) { free_pending_exception(pe); @@ -1810,7 +1808,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) bio->bi_iter.bi_size == (s->store->chunk_size << SECTOR_SHIFT)) { pe->started = 1; - mutex_unlock(&s->lock); + up_write(&s->lock); start_full_bio(pe, bio); goto out; } @@ -1820,7 +1818,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) if (!pe->started) { /* this is protected by snap->lock */ pe->started = 1; - mutex_unlock(&s->lock); + up_write(&s->lock); start_copy(pe); goto out; } @@ -1830,7 +1828,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) } out_unlock: - mutex_unlock(&s->lock); + up_write(&s->lock); out: return r; } @@ -1866,7 +1864,7 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio) chunk = sector_to_chunk(s->store, bio->bi_iter.bi_sector); - mutex_lock(&s->lock); + down_write(&s->lock); /* Full merging snapshots are redirected to the origin */ if (!s->valid) @@ -1897,12 +1895,12 @@ redirect_to_origin: bio_set_dev(bio, s->origin->bdev); if (bio_data_dir(bio) == WRITE) { - mutex_unlock(&s->lock); + up_write(&s->lock); return do_origin(s->origin, bio); } out_unlock: - mutex_unlock(&s->lock); + up_write(&s->lock); return r; } @@ -1934,7 +1932,7 @@ static int snapshot_preresume(struct dm_target *ti) down_read(&_origins_lock); (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL); if (snap_src && snap_dest) { - mutex_lock(&snap_src->lock); + down_read(&snap_src->lock); if (s == snap_src) { DMERR("Unable to resume snapshot source until " "handover completes."); @@ -1944,7 +1942,7 @@ static int snapshot_preresume(struct dm_target *ti) "source is suspended."); r = -EINVAL; } - mutex_unlock(&snap_src->lock); + up_read(&snap_src->lock); } up_read(&_origins_lock); @@ -1990,11 +1988,11 @@ static void snapshot_resume(struct dm_target *ti) (void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL); if (snap_src && snap_dest) { - mutex_lock(&snap_src->lock); - mutex_lock_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING); + down_write(&snap_src->lock); + down_write_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING); __handover_exceptions(snap_src, snap_dest); - mutex_unlock(&snap_dest->lock); - mutex_unlock(&snap_src->lock); + up_write(&snap_dest->lock); + up_write(&snap_src->lock); } up_read(&_origins_lock); @@ -2009,9 +2007,9 @@ static void snapshot_resume(struct dm_target *ti) /* Now we have correct chunk size, reregister */ reregister_snapshot(s); - mutex_lock(&s->lock); + down_write(&s->lock); s->active = 1; - mutex_unlock(&s->lock); + up_write(&s->lock); } static uint32_t get_origin_minimum_chunksize(struct block_device *bdev) @@ -2051,7 +2049,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type, switch (type) { case STATUSTYPE_INFO: - mutex_lock(&snap->lock); + down_write(&snap->lock); if (!snap->valid) DMEMIT("Invalid"); @@ -2076,7 +2074,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type, DMEMIT("Unknown"); } - mutex_unlock(&snap->lock); + up_write(&snap->lock); break; @@ -2142,7 +2140,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, if (dm_target_is_snapshot_merge(snap->ti)) continue; - mutex_lock(&snap->lock); + down_write(&snap->lock); /* Only deal with valid and active snapshots */ if (!snap->valid || !snap->active) @@ -2169,9 +2167,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, if (e) goto next_snapshot; - mutex_unlock(&snap->lock); + up_write(&snap->lock); pe = alloc_pending_exception(snap); - mutex_lock(&snap->lock); + down_write(&snap->lock); if (!snap->valid) { free_pending_exception(pe); @@ -2221,7 +2219,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, } next_snapshot: - mutex_unlock(&snap->lock); + up_write(&snap->lock); if (pe_to_start_now) { start_copy(pe_to_start_now); -- 2.30.2