[PATCH v3.1 9/9] btrfs: qgroup: Don't scan leaf if we're modifying reloc tree

[Qu Wenruo <wqu@suse.com>]

Since reloc tree doesn't contribute to qgroup numbers, just skip them.

This should catch the final cause of unnecessary data refs for qgroup +
metadata balance.

The 4G data 16 snapshots test (*) should explain it pretty well:

             | delayed subtree | refactor delayed ref | this patch
---------------------------------------------------------------------
relocated    |           22653 |                22673 |         22744
qgroup dirty |          122792 |                48360 |            70
time         |          24.494 |               11.606 |         3.944

Finally, we're at the stage where qgroup + metadata balance cost no
obvious overhead.

*: Test environment
Test VM:
- vRAM		8G
- vCPU		8
- block dev	vitrio-blk, 'unsafe' cache mode
- host block	850evo

Test workload
- Copy 4G data from /usr/ to one subvolume
- Create 16 snapshots of that subvolume, and modify 3 files in each
  snapshot
- Enable quota, rescan
- Time "btrfs balance start -m"

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent-tree.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 2c08b9646c61..2d71cc1acb2f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -9235,11 +9235,13 @@ static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
 			else
 				ret = btrfs_dec_ref(trans, root, eb, 0);
 			BUG_ON(ret); /* -ENOMEM */
-			ret = btrfs_qgroup_trace_leaf_items(trans, eb);
-			if (ret) {
-				btrfs_err_rl(fs_info,
-					     "error %d accounting leaf items. Quota is out of sync, rescan required.",
+			if (is_fstree(root->root_key.objectid)) {
+				ret = btrfs_qgroup_trace_leaf_items(trans, eb);
+				if (ret) {
+					btrfs_err_rl(fs_info,
+"error %d accounting leaf items. Quota is out of sync, rescan required.",
 					     ret);
+				}
 			}
 		}
 		/* make block locked assertion in btrfs_clean_tree_block happy */
-- 
2.21.0