btrfs: do not block starts waiting on previous transaction commit

Internally I got a report of very long stalls on normal operations like
creating a new file when auto relocation was running.  The reporter used
the 'bpf offcputime' tracer to show that we would get stuck in
start_transaction for 5 to 30 seconds, and were always being woken up by
the transaction commit.

Using my timing-everything script, which times how long a function takes
and what percentage of that total time is taken up by its children, I
saw several traces like this

1083 took 32812902424 ns
        29929002926 ns 91.2110% wait_for_commit_duration
        25568 ns 7.7920e-05% commit_fs_roots_duration
        1007751 ns 0.00307% commit_cowonly_roots_duration
        446855602 ns 1.36182% btrfs_run_delayed_refs_duration
        271980 ns 0.00082% btrfs_run_delayed_items_duration
        2008 ns 6.1195e-06% btrfs_apply_pending_changes_duration
        9656 ns 2.9427e-05% switch_commit_roots_duration
        1598 ns 4.8700e-06% btrfs_commit_device_sizes_duration
        4314 ns 1.3147e-05% btrfs_free_log_root_tree_duration

Here I was only tracing functions that happen where we are between
START_COMMIT and UNBLOCKED in order to see what would be keeping us
blocked for so long.  The wait_for_commit() we do is where we wait for a
previous transaction that hasn't completed it's commit.  This can
include all of the unpin work and other cleanups, which tends to be the
longest part of our transaction commit.

There is no reason we should be blocking new things from entering the
transaction at this point, it just adds to random latency spikes for no
reason.

Fix this by adding a PREP stage.  This allows us to properly deal with
multiple committers coming in at the same time, we retain the behavior
that the winner waits on the previous transaction and the losers all
wait for this transaction commit to occur.  Nothing else is blocked
during the PREP stage, and then once the wait is complete we switch to
COMMIT_START and all of the same behavior as before is maintained.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1547,7 +1547,7 @@ static int transaction_kthread(void *arg)
 
 		delta = ktime_get_seconds() - cur->start_time;
 		if (!test_and_clear_bit(BTRFS_FS_COMMIT_TRANS, &fs_info->flags) &&
-		    cur->state < TRANS_STATE_COMMIT_START &&
+		    cur->state < TRANS_STATE_COMMIT_PREP &&
 		    delta < fs_info->commit_interval) {
 			spin_unlock(&fs_info->trans_lock);
 			delay -= msecs_to_jiffies((delta - 1) * 1000);
@@ -2682,8 +2682,8 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 	btrfs_lockdep_init_map(fs_info, btrfs_trans_num_extwriters);
 	btrfs_lockdep_init_map(fs_info, btrfs_trans_pending_ordered);
 	btrfs_lockdep_init_map(fs_info, btrfs_ordered_extent);
-	btrfs_state_lockdep_init_map(fs_info, btrfs_trans_commit_start,
-				     BTRFS_LOCKDEP_TRANS_COMMIT_START);
+	btrfs_state_lockdep_init_map(fs_info, btrfs_trans_commit_prep,
+				     BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 	btrfs_state_lockdep_init_map(fs_info, btrfs_trans_unblocked,
 				     BTRFS_LOCKDEP_TRANS_UNBLOCKED);
 	btrfs_state_lockdep_init_map(fs_info, btrfs_trans_super_committed,
@@ -4870,7 +4870,7 @@ static int btrfs_cleanup_transaction(struct btrfs_fs_info *fs_info)
 	while (!list_empty(&fs_info->trans_list)) {
 		t = list_first_entry(&fs_info->trans_list,
 				     struct btrfs_transaction, list);
-		if (t->state >= TRANS_STATE_COMMIT_START) {
+		if (t->state >= TRANS_STATE_COMMIT_PREP) {
 			refcount_inc(&t->use_count);
 			spin_unlock(&fs_info->trans_lock);
 			btrfs_wait_for_commit(fs_info, t->transid);

fs/btrfs/locking.h

@@ -79,7 +79,7 @@ enum btrfs_lock_nesting {
 };
 
 enum btrfs_lockdep_trans_states {
-	BTRFS_LOCKDEP_TRANS_COMMIT_START,
+	BTRFS_LOCKDEP_TRANS_COMMIT_PREP,
 	BTRFS_LOCKDEP_TRANS_UNBLOCKED,
 	BTRFS_LOCKDEP_TRANS_SUPER_COMMITTED,
 	BTRFS_LOCKDEP_TRANS_COMPLETED,

fs/btrfs/transaction.c

@@ -56,12 +56,17 @@ static struct kmem_cache *btrfs_trans_handle_cachep;
  * |  Call btrfs_commit_transaction() on any trans handle attached to
  * |  transaction N
  * V
- * Transaction N [[TRANS_STATE_COMMIT_START]]
+ * Transaction N [[TRANS_STATE_COMMIT_PREP]]
+ * |
+ * | If there are simultaneous calls to btrfs_commit_transaction() one will win
+ * | the race and the rest will wait for the winner to commit the transaction.
  * |
- * | Will wait for previous running transaction to completely finish if there
- * | is one
+ * | The winner will wait for previous running transaction to completely finish
+ * | if there is one.
+ * |
+ * Transaction N [[TRANS_STATE_COMMIT_START]]
  * |
- * | Then one of the following happes:
+ * | Then one of the following happens:
  * | - Wait for all other trans handle holders to release.
  * |   The btrfs_commit_transaction() caller will do the commit work.
  * | - Wait for current transaction to be committed by others.
@@ -112,6 +117,7 @@ static struct kmem_cache *btrfs_trans_handle_cachep;
  */
 static const unsigned int btrfs_blocked_trans_types[TRANS_STATE_MAX] = {
 	[TRANS_STATE_RUNNING]		= 0U,
+	[TRANS_STATE_COMMIT_PREP]	= 0U,
 	[TRANS_STATE_COMMIT_START]	= (__TRANS_START | __TRANS_ATTACH),
 	[TRANS_STATE_COMMIT_DOING]	= (__TRANS_START |
 					   __TRANS_ATTACH |
@@ -1983,7 +1989,7 @@ void btrfs_commit_transaction_async(struct btrfs_trans_handle *trans)
 	 * Wait for the current transaction commit to start and block
 	 * subsequent transaction joins
 	 */
-	btrfs_might_wait_for_state(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_START);
+	btrfs_might_wait_for_state(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 	wait_event(fs_info->transaction_blocked_wait,
 		   cur_trans->state >= TRANS_STATE_COMMIT_START ||
 		   TRANS_ABORTED(cur_trans));
@@ -2130,7 +2136,7 @@ static void add_pending_snapshot(struct btrfs_trans_handle *trans)
 		return;
 
 	lockdep_assert_held(&trans->fs_info->trans_lock);
-	ASSERT(cur_trans->state >= TRANS_STATE_COMMIT_START);
+	ASSERT(cur_trans->state >= TRANS_STATE_COMMIT_PREP);
 
 	list_add(&trans->pending_snapshot->list, &cur_trans->pending_snapshots);
 }
@@ -2154,7 +2160,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	ktime_t interval;
 
 	ASSERT(refcount_read(&trans->use_count) == 1);
-	btrfs_trans_state_lockdep_acquire(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_START);
+	btrfs_trans_state_lockdep_acquire(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 
 	clear_bit(BTRFS_FS_NEED_TRANS_COMMIT, &fs_info->flags);
 
@@ -2214,7 +2220,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	}
 
 	spin_lock(&fs_info->trans_lock);
-	if (cur_trans->state >= TRANS_STATE_COMMIT_START) {
+	if (cur_trans->state >= TRANS_STATE_COMMIT_PREP) {
 		enum btrfs_trans_state want_state = TRANS_STATE_COMPLETED;
 
 		add_pending_snapshot(trans);
@@ -2226,7 +2232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 			want_state = TRANS_STATE_SUPER_COMMITTED;
 
 		btrfs_trans_state_lockdep_release(fs_info,
-						  BTRFS_LOCKDEP_TRANS_COMMIT_START);
+						  BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 		ret = btrfs_end_transaction(trans);
 		wait_for_commit(cur_trans, want_state);
 
@@ -2238,9 +2244,9 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 		return ret;
 	}
 
-	cur_trans->state = TRANS_STATE_COMMIT_START;
+	cur_trans->state = TRANS_STATE_COMMIT_PREP;
 	wake_up(&fs_info->transaction_blocked_wait);
-	btrfs_trans_state_lockdep_release(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_START);
+	btrfs_trans_state_lockdep_release(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 
 	if (cur_trans->list.prev != &fs_info->trans_list) {
 		enum btrfs_trans_state want_state = TRANS_STATE_COMPLETED;
@@ -2261,11 +2267,9 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 			btrfs_put_transaction(prev_trans);
 			if (ret)
 				goto lockdep_release;
-		} else {
-			spin_unlock(&fs_info->trans_lock);
+			spin_lock(&fs_info->trans_lock);
 		}
 	} else {
-		spin_unlock(&fs_info->trans_lock);
 		/*
 		 * The previous transaction was aborted and was already removed
 		 * from the list of transactions at fs_info->trans_list. So we
@@ -2273,11 +2277,16 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 		 * corrupt state (pointing to trees with unwritten nodes/leafs).
 		 */
 		if (BTRFS_FS_ERROR(fs_info)) {
+			spin_unlock(&fs_info->trans_lock);
 			ret = -EROFS;
 			goto lockdep_release;
 		}
 	}
 
+	cur_trans->state = TRANS_STATE_COMMIT_START;
+	wake_up(&fs_info->transaction_blocked_wait);
+	spin_unlock(&fs_info->trans_lock);
+
 	/*
 	 * Get the time spent on the work done by the commit thread and not
 	 * the time spent waiting on a previous commit
@@ -2587,7 +2596,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	goto cleanup_transaction;
 
 lockdep_trans_commit_start_release:
-	btrfs_trans_state_lockdep_release(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_START);
+	btrfs_trans_state_lockdep_release(fs_info, BTRFS_LOCKDEP_TRANS_COMMIT_PREP);
 	btrfs_end_transaction(trans);
 	return ret;
 }

fs/btrfs/transaction.h

@@ -14,6 +14,7 @@
 
 enum btrfs_trans_state {
 	TRANS_STATE_RUNNING,
+	TRANS_STATE_COMMIT_PREP,
 	TRANS_STATE_COMMIT_START,
 	TRANS_STATE_COMMIT_DOING,
 	TRANS_STATE_UNBLOCKED,