btrfs: qgroup: Introduce trace event to analyse the number of dirty extents accounted

Number of qgroup dirty extents is directly linked to the performance
overhead, so add a new trace event, trace_qgroup_num_dirty_extents(), to
record how many dirty extents is processed in
btrfs_qgroup_account_extents().

This will be pretty handy to analyze later balance performance
improvement.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -2132,6 +2132,7 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans)
 	struct btrfs_delayed_ref_root *delayed_refs;
 	struct ulist *new_roots = NULL;
 	struct rb_node *node;
+	u64 num_dirty_extents = 0;
 	u64 qgroup_to_skip;
 	int ret = 0;
 
@@ -2141,6 +2142,7 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans)
 		record = rb_entry(node, struct btrfs_qgroup_extent_record,
 				  node);
 
+		num_dirty_extents++;
 		trace_btrfs_qgroup_account_extents(fs_info, record);
 
 		if (!ret) {
@@ -2186,6 +2188,8 @@ cleanup:
 		kfree(record);
 
 	}
+	trace_qgroup_num_dirty_extents(fs_info, trans->transid,
+				       num_dirty_extents);
 	return ret;
 }