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Commit 3f1637f2 authored by Nikos Tsironis's avatar Nikos Tsironis Committed by Mike Snitzer
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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-suiteCo-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
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Inline Side-by-side
Showing with 44 additions and 40 deletions
drivers/md/dm-snap.c
View file @ 3f1637f2
......@@ -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);
......
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