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
Mike Snitzer
Родитель
65fc7c3704
Коммит
4ad8d880b6
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@ -48,7 +48,7 @@ struct dm_exception_table {
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};
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struct dm_snapshot {
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struct mutex lock;
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struct rw_semaphore lock;
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struct dm_dev *origin;
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struct dm_dev *cow;
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@ -457,9 +457,9 @@ static int __find_snapshots_sharing_cow(struct dm_snapshot *snap,
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if (!bdev_equal(s->cow->bdev, snap->cow->bdev))
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continue;
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mutex_lock(&s->lock);
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down_read(&s->lock);
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active = s->active;
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mutex_unlock(&s->lock);
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up_read(&s->lock);
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if (active) {
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if (snap_src)
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@ -927,7 +927,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s)
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int r;
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chunk_t old_chunk = s->first_merging_chunk + s->num_merging_chunks - 1;
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mutex_lock(&s->lock);
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down_write(&s->lock);
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/*
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* Process chunks (and associated exceptions) in reverse order
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@ -942,7 +942,7 @@ static int remove_single_exception_chunk(struct dm_snapshot *s)
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b = __release_queued_bios_after_merge(s);
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out:
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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if (b)
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flush_bios(b);
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@ -1001,9 +1001,9 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
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if (linear_chunks < 0) {
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DMERR("Read error in exception store: "
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"shutting down merge");
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mutex_lock(&s->lock);
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down_write(&s->lock);
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s->merge_failed = 1;
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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}
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goto shut;
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}
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@ -1044,10 +1044,10 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
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previous_count = read_pending_exceptions_done_count();
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}
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mutex_lock(&s->lock);
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down_write(&s->lock);
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s->first_merging_chunk = old_chunk;
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s->num_merging_chunks = linear_chunks;
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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/* Wait until writes to all 'linear_chunks' drain */
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for (i = 0; i < linear_chunks; i++)
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@ -1089,10 +1089,10 @@ static void merge_callback(int read_err, unsigned long write_err, void *context)
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return;
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shut:
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mutex_lock(&s->lock);
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down_write(&s->lock);
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s->merge_failed = 1;
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b = __release_queued_bios_after_merge(s);
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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error_bios(b);
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merge_shutdown(s);
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@ -1191,7 +1191,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
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s->exception_start_sequence = 0;
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s->exception_complete_sequence = 0;
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s->out_of_order_tree = RB_ROOT;
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mutex_init(&s->lock);
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init_rwsem(&s->lock);
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INIT_LIST_HEAD(&s->list);
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spin_lock_init(&s->pe_lock);
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s->state_bits = 0;
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@ -1357,9 +1357,9 @@ static void snapshot_dtr(struct dm_target *ti)
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/* Check whether exception handover must be cancelled */
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(void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
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if (snap_src && snap_dest && (s == snap_src)) {
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mutex_lock(&snap_dest->lock);
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down_write(&snap_dest->lock);
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snap_dest->valid = 0;
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mutex_unlock(&snap_dest->lock);
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up_write(&snap_dest->lock);
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DMERR("Cancelling snapshot handover.");
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}
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up_read(&_origins_lock);
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@ -1390,8 +1390,6 @@ static void snapshot_dtr(struct dm_target *ti)
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dm_exception_store_destroy(s->store);
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mutex_destroy(&s->lock);
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dm_put_device(ti, s->cow);
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dm_put_device(ti, s->origin);
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@ -1479,7 +1477,7 @@ static void pending_complete(void *context, int success)
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if (!success) {
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/* Read/write error - snapshot is unusable */
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mutex_lock(&s->lock);
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down_write(&s->lock);
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__invalidate_snapshot(s, -EIO);
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error = 1;
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goto out;
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@ -1487,14 +1485,14 @@ static void pending_complete(void *context, int success)
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e = alloc_completed_exception(GFP_NOIO);
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if (!e) {
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mutex_lock(&s->lock);
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down_write(&s->lock);
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__invalidate_snapshot(s, -ENOMEM);
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error = 1;
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goto out;
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}
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*e = pe->e;
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mutex_lock(&s->lock);
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down_write(&s->lock);
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if (!s->valid) {
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free_completed_exception(e);
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error = 1;
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@ -1512,9 +1510,9 @@ static void pending_complete(void *context, int success)
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/* Wait for conflicting reads to drain */
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if (__chunk_is_tracked(s, pe->e.old_chunk)) {
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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__check_for_conflicting_io(s, pe->e.old_chunk);
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mutex_lock(&s->lock);
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down_write(&s->lock);
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}
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out:
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@ -1527,7 +1525,7 @@ out:
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full_bio->bi_end_io = pe->full_bio_end_io;
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increment_pending_exceptions_done_count();
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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/* Submit any pending write bios */
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if (error) {
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@ -1750,7 +1748,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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if (!s->valid)
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return DM_MAPIO_KILL;
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mutex_lock(&s->lock);
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down_write(&s->lock);
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if (!s->valid || (unlikely(s->snapshot_overflowed) &&
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bio_data_dir(bio) == WRITE)) {
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@ -1773,9 +1771,9 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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if (bio_data_dir(bio) == WRITE) {
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pe = __lookup_pending_exception(s, chunk);
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if (!pe) {
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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pe = alloc_pending_exception(s);
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mutex_lock(&s->lock);
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down_write(&s->lock);
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if (!s->valid || s->snapshot_overflowed) {
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free_pending_exception(pe);
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@ -1810,7 +1808,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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bio->bi_iter.bi_size ==
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(s->store->chunk_size << SECTOR_SHIFT)) {
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pe->started = 1;
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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start_full_bio(pe, bio);
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goto out;
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}
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@ -1820,7 +1818,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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if (!pe->started) {
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/* this is protected by snap->lock */
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pe->started = 1;
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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start_copy(pe);
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goto out;
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}
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@ -1830,7 +1828,7 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
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}
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out_unlock:
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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out:
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return r;
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}
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@ -1866,7 +1864,7 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio)
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chunk = sector_to_chunk(s->store, bio->bi_iter.bi_sector);
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mutex_lock(&s->lock);
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down_write(&s->lock);
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/* Full merging snapshots are redirected to the origin */
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if (!s->valid)
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@ -1897,12 +1895,12 @@ redirect_to_origin:
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bio_set_dev(bio, s->origin->bdev);
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if (bio_data_dir(bio) == WRITE) {
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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return do_origin(s->origin, bio);
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}
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out_unlock:
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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return r;
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}
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@ -1934,7 +1932,7 @@ static int snapshot_preresume(struct dm_target *ti)
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down_read(&_origins_lock);
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(void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
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if (snap_src && snap_dest) {
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mutex_lock(&snap_src->lock);
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down_read(&snap_src->lock);
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if (s == snap_src) {
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DMERR("Unable to resume snapshot source until "
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"handover completes.");
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@ -1944,7 +1942,7 @@ static int snapshot_preresume(struct dm_target *ti)
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"source is suspended.");
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r = -EINVAL;
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}
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mutex_unlock(&snap_src->lock);
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up_read(&snap_src->lock);
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}
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up_read(&_origins_lock);
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@ -1990,11 +1988,11 @@ static void snapshot_resume(struct dm_target *ti)
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(void) __find_snapshots_sharing_cow(s, &snap_src, &snap_dest, NULL);
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if (snap_src && snap_dest) {
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mutex_lock(&snap_src->lock);
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mutex_lock_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
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down_write(&snap_src->lock);
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down_write_nested(&snap_dest->lock, SINGLE_DEPTH_NESTING);
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__handover_exceptions(snap_src, snap_dest);
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mutex_unlock(&snap_dest->lock);
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mutex_unlock(&snap_src->lock);
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up_write(&snap_dest->lock);
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up_write(&snap_src->lock);
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}
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up_read(&_origins_lock);
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@ -2009,9 +2007,9 @@ static void snapshot_resume(struct dm_target *ti)
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/* Now we have correct chunk size, reregister */
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reregister_snapshot(s);
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mutex_lock(&s->lock);
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down_write(&s->lock);
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s->active = 1;
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mutex_unlock(&s->lock);
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up_write(&s->lock);
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}
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static uint32_t get_origin_minimum_chunksize(struct block_device *bdev)
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@ -2051,7 +2049,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type,
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switch (type) {
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case STATUSTYPE_INFO:
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mutex_lock(&snap->lock);
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down_write(&snap->lock);
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if (!snap->valid)
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DMEMIT("Invalid");
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@ -2076,7 +2074,7 @@ static void snapshot_status(struct dm_target *ti, status_type_t type,
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DMEMIT("Unknown");
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}
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mutex_unlock(&snap->lock);
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up_write(&snap->lock);
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break;
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@ -2142,7 +2140,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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if (dm_target_is_snapshot_merge(snap->ti))
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continue;
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mutex_lock(&snap->lock);
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down_write(&snap->lock);
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/* Only deal with valid and active snapshots */
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if (!snap->valid || !snap->active)
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@ -2169,9 +2167,9 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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if (e)
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goto next_snapshot;
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mutex_unlock(&snap->lock);
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up_write(&snap->lock);
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pe = alloc_pending_exception(snap);
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mutex_lock(&snap->lock);
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down_write(&snap->lock);
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if (!snap->valid) {
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free_pending_exception(pe);
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@ -2221,7 +2219,7 @@ static int __origin_write(struct list_head *snapshots, sector_t sector,
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}
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next_snapshot:
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mutex_unlock(&snap->lock);
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up_write(&snap->lock);
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if (pe_to_start_now) {
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start_copy(pe_to_start_now);
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