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Subject[PATCH v3 6/6] dm snapshot: Use fine-grained locking scheme
Date
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@arrikto.com>
Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
---
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
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