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Commit 3f1637f2 authored by Nikos Tsironis's avatar Nikos Tsironis Committed by Mike Snitzer
Browse files

dm snapshot: Use fine-grained locking scheme 

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



Co-developed-by: default avatarIlias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: default avatarNikos Tsironis <ntsironis@arrikto.com>
Acked-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent f79ae415

drivers/md/dm-snap.c

+44 −40
Original line number Diff line number Diff line
@@ -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) {

					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);