Substitute the global locking scheme with a fine grained one, employing the read-write semaphore and the scalable exception tables with per-bucket locks introduced by the previous two commits. Summarizing, we now 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. Finally, we use an extra spinlock (pe_allocation_lock) to serialize the allocation of new exceptions by the exception store. This allocation is really fast, so the extra spinlock doesn't hurt the performance. This scheme allows dm-snapshot to scale better, resulting in increased IOPS and reduced latency. Following are some benchmark results using the null_blk device: modprobe null_blk gb=1024 bs=512 submit_queues=8 hw_queue_depth=4096 \ queue_mode=2 irqmode=1 completion_nsec=1 nr_devices=1 * Benchmark fio_origin_randwrite_throughput_N, from the device mapper test suite [1] (direct IO, random 4K writes to origin device, IO engine libaio): +--------------+-------------+------------+ | # of workers | IOPS Before | IOPS After | +--------------+-------------+------------+ | 1 | 57708 | 66421 | | 2 | 63415 | 77589 | | 4 | 67276 | 98839 | | 8 | 60564 | 109258 | +--------------+-------------+------------+ * Benchmark fio_origin_randwrite_latency_N, from the device mapper test suite [1] (direct IO, random 4K writes to origin device, IO engine psync): +--------------+-----------------------+----------------------+ | # of workers | Latency (usec) Before | Latency (usec) After | +--------------+-----------------------+----------------------+ | 1 | 16.25 | 13.27 | | 2 | 31.65 | 25.08 | | 4 | 55.28 | 41.08 | | 8 | 121.47 | 74.44 | +--------------+-----------------------+----------------------+ * Benchmark fio_snapshot_randwrite_throughput_N, from the device mapper test suite [1] (direct IO, random 4K writes to snapshot device, IO engine libaio): +--------------+-------------+------------+ | # of workers | IOPS Before | IOPS After | +--------------+-------------+------------+ | 1 | 72593 | 84938 | | 2 | 97379 | 134973 | | 4 | 90610 | 143077 | | 8 | 90537 | 180085 | +--------------+-------------+------------+ * Benchmark fio_snapshot_randwrite_latency_N, from the device mapper test suite [1] (direct IO, random 4K writes to snapshot device, IO engine psync): +--------------+-----------------------+----------------------+ | # of workers | Latency (usec) Before | Latency (usec) After | +--------------+-----------------------+----------------------+ | 1 | 12.53 | 10.6 | | 2 | 19.78 | 14.89 | | 4 | 40.37 | 23.47 | | 8 | 89.32 | 48.48 | +--------------+-----------------------+----------------------+ [1] https://github.com/jthornber/device-mapper-test-suite Signed-off-by: Nikos Tsironis <ntsironis@xxxxxxxxxxx> Signed-off-by: Ilias Tsitsimpis <iliastsi@xxxxxxxxxxx> --- drivers/md/dm-snap.c | 84 +++++++++++++++++++++++++++------------------------- 1 file changed, 44 insertions(+), 40 deletions(-) diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c index ab75857309aa..4530d0620a72 100644 --- a/drivers/md/dm-snap.c +++ b/drivers/md/dm-snap.c @@ -77,7 +77,9 @@ struct dm_snapshot { atomic_t pending_exceptions_count; - /* Protected by "lock" */ + spinlock_t pe_allocation_lock; + + /* Protected by "pe_allocation_lock" */ sector_t exception_start_sequence; /* Protected by kcopyd single-threaded callback */ @@ -1245,6 +1247,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv) s->snapshot_overflowed = 0; s->active = 0; atomic_set(&s->pending_exceptions_count, 0); + spin_lock_init(&s->pe_allocation_lock); s->exception_start_sequence = 0; s->exception_complete_sequence = 0; s->out_of_order_tree = RB_ROOT; @@ -1522,6 +1525,13 @@ static void __invalidate_snapshot(struct dm_snapshot *s, int err) dm_table_event(s->ti->table); } +static void invalidate_snapshot(struct dm_snapshot *s, int err) +{ + down_write(&s->lock); + __invalidate_snapshot(s, err); + up_write(&s->lock); +} + static void pending_complete(void *context, int success) { struct dm_snap_pending_exception *pe = context; @@ -1537,8 +1547,7 @@ static void pending_complete(void *context, int success) if (!success) { /* Read/write error - snapshot is unusable */ - down_write(&s->lock); - __invalidate_snapshot(s, -EIO); + invalidate_snapshot(s, -EIO); error = 1; dm_exception_table_lock(&lock); @@ -1547,8 +1556,7 @@ static void pending_complete(void *context, int success) e = alloc_completed_exception(GFP_NOIO); if (!e) { - down_write(&s->lock); - __invalidate_snapshot(s, -ENOMEM); + invalidate_snapshot(s, -ENOMEM); error = 1; dm_exception_table_lock(&lock); @@ -1556,11 +1564,13 @@ static void pending_complete(void *context, int success) } *e = pe->e; - down_write(&s->lock); + down_read(&s->lock); dm_exception_table_lock(&lock); if (!s->valid) { + up_read(&s->lock); free_completed_exception(e); error = 1; + goto out; } @@ -1572,13 +1582,12 @@ static void pending_complete(void *context, int success) * merging can overwrite the chunk in origin. */ dm_insert_exception(&s->complete, e); + up_read(&s->lock); /* Wait for conflicting reads to drain */ if (__chunk_is_tracked(s, pe->e.old_chunk)) { dm_exception_table_unlock(&lock); - up_write(&s->lock); __check_for_conflicting_io(s, pe->e.old_chunk); - down_write(&s->lock); dm_exception_table_lock(&lock); } @@ -1595,8 +1604,6 @@ static void pending_complete(void *context, int success) full_bio->bi_end_io = pe->full_bio_end_io; increment_pending_exceptions_done_count(); - up_write(&s->lock); - /* Submit any pending write bios */ if (error) { if (full_bio) @@ -1738,8 +1745,8 @@ __lookup_pending_exception(struct dm_snapshot *s, chunk_t chunk) /* * Inserts a pending exception into the pending table. * - * NOTE: a write lock must be held on snap->lock before calling - * this. + * NOTE: a write lock must be held on the chunk's pending exception table slot + * before calling this. */ static struct dm_snap_pending_exception * __insert_pending_exception(struct dm_snapshot *s, @@ -1751,12 +1758,15 @@ __insert_pending_exception(struct dm_snapshot *s, pe->started = 0; pe->full_bio = NULL; + spin_lock(&s->pe_allocation_lock); if (s->store->type->prepare_exception(s->store, &pe->e)) { + spin_unlock(&s->pe_allocation_lock); free_pending_exception(pe); return NULL; } pe->exception_sequence = s->exception_start_sequence++; + spin_unlock(&s->pe_allocation_lock); dm_insert_exception(&s->pending, &pe->e); @@ -1768,8 +1778,8 @@ __insert_pending_exception(struct dm_snapshot *s, * for this chunk, otherwise it allocates a new one and inserts * it into the pending table. * - * NOTE: a write lock must be held on snap->lock before calling - * this. + * NOTE: a write lock must be held on the chunk's pending exception table slot + * before calling this. */ static struct dm_snap_pending_exception * __find_pending_exception(struct dm_snapshot *s, @@ -1820,7 +1830,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) if (!s->valid) return DM_MAPIO_KILL; - down_write(&s->lock); + down_read(&s->lock); dm_exception_table_lock(&lock); if (!s->valid || (unlikely(s->snapshot_overflowed) && @@ -1845,17 +1855,9 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) pe = __lookup_pending_exception(s, chunk); if (!pe) { dm_exception_table_unlock(&lock); - up_write(&s->lock); pe = alloc_pending_exception(s); - down_write(&s->lock); dm_exception_table_lock(&lock); - if (!s->valid || s->snapshot_overflowed) { - free_pending_exception(pe); - r = DM_MAPIO_KILL; - goto out_unlock; - } - e = dm_lookup_exception(&s->complete, chunk); if (e) { free_pending_exception(pe); @@ -1866,10 +1868,15 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) pe = __find_pending_exception(s, pe, chunk); if (!pe) { dm_exception_table_unlock(&lock); + up_read(&s->lock); + + down_write(&s->lock); if (s->store->userspace_supports_overflow) { - s->snapshot_overflowed = 1; - DMERR("Snapshot overflowed: Unable to allocate exception."); + if (s->valid && !s->snapshot_overflowed) { + s->snapshot_overflowed = 1; + DMERR("Snapshot overflowed: Unable to allocate exception."); + } } else __invalidate_snapshot(s, -ENOMEM); up_write(&s->lock); @@ -1887,8 +1894,10 @@ 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; + dm_exception_table_unlock(&lock); - up_write(&s->lock); + up_read(&s->lock); + start_full_bio(pe, bio); goto out; } @@ -1896,10 +1905,12 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) bio_list_add(&pe->snapshot_bios, bio); if (!pe->started) { - /* this is protected by snap->lock */ + /* this is protected by the exception table lock */ pe->started = 1; + dm_exception_table_unlock(&lock); - up_write(&s->lock); + up_read(&s->lock); + start_copy(pe); goto out; } @@ -1910,7 +1921,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) out_unlock: dm_exception_table_unlock(&lock); - up_write(&s->lock); + up_read(&s->lock); out: return r; } @@ -2234,7 +2245,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, chunk = sector_to_chunk(snap->store, sector); dm_exception_table_lock_init(snap, chunk, &lock); - down_write(&snap->lock); + down_read(&snap->lock); dm_exception_table_lock(&lock); /* Only deal with valid and active snapshots */ @@ -2253,16 +2264,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, goto next_snapshot; dm_exception_table_unlock(&lock); - up_write(&snap->lock); pe = alloc_pending_exception(snap); - down_write(&snap->lock); dm_exception_table_lock(&lock); - if (!snap->valid) { - free_pending_exception(pe); - goto next_snapshot; - } - pe2 = __lookup_pending_exception(snap, chunk); if (!pe2) { @@ -2275,9 +2279,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, pe = __insert_pending_exception(snap, pe, chunk); if (!pe) { dm_exception_table_unlock(&lock); - __invalidate_snapshot(snap, -ENOMEM); - up_write(&snap->lock); + up_read(&snap->lock); + invalidate_snapshot(snap, -ENOMEM); continue; } } else { @@ -2310,7 +2314,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, next_snapshot: dm_exception_table_unlock(&lock); - up_write(&snap->lock); + up_read(&snap->lock); if (pe_to_start_now) { start_copy(pe_to_start_now); -- 2.11.0 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
