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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 ae1093be5a ("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 <iliastsi@arrikto.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Nikos Tsironis 2019-03-17 14:22:56 +02:00 committed by Mike Snitzer
parent 65fc7c3704
commit 4ad8d880b6
1 changed files with 43 additions and 45 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

88
drivers/md/dm-snap.c
View File

@ -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 @@ static void pending_complete(void *context, int success)
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 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio)
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);