btrfs: qgroup: Use delayed subtree rescan for balance

Before this patch, qgroup code traces the whole subtree of subvolume and
reloc trees unconditionally.

This makes qgroup numbers consistent, but it could cause tons of
unnecessary extent tracing, which causes a lot of overhead.

However for subtree swap of balance, just swap both subtrees because
they contain the same contents and tree structure, so qgroup numbers
won't change.

It's the race window between subtree swap and transaction commit could
cause qgroup number change.

This patch will delay the qgroup subtree scan until COW happens for the
subtree root.

So if there is no other operations for the fs, balance won't cause extra
qgroup overhead. (best case scenario)
Depending on the workload, most of the subtree scan can still be
avoided.

Only for worst case scenario, it will fall back to old subtree swap
overhead. (scan all swapped subtrees)

[[Benchmark]]
Hardware:
	VM 4G vRAM, 8 vCPUs,
	disk is using 'unsafe' cache mode,
	backing device is SAMSUNG 850 evo SSD.
	Host has 16G ram.

Mkfs parameter:
	--nodesize 4K (To bump up tree size)

Initial subvolume contents:
	4G data copied from /usr and /lib.
	(With enough regular small files)

Snapshots:
	16 snapshots of the original subvolume.
	each snapshot has 3 random files modified.

balance parameter:
	-m

So the content should be pretty similar to a real world root fs layout.

And after file system population, there is no other activity, so it
should be the best case scenario.

                     | v4.20-rc1            | w/ patchset    | diff
-----------------------------------------------------------------------
relocated extents    | 22615                | 22457          | -0.1%
qgroup dirty extents | 163457               | 121606         | -25.6%
time (sys)           | 22.884s              | 18.842s        | -17.6%
time (real)          | 27.724s              | 22.884s        | -17.5%

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

fs/btrfs/ctree.c

@@ -13,6 +13,7 @@
 #include "print-tree.h"
 #include "locking.h"
 #include "volumes.h"
+#include "qgroup.h"
 
 static int split_node(struct btrfs_trans_handle *trans, struct btrfs_root
 		      *root, struct btrfs_path *path, int level);
@@ -1489,6 +1490,13 @@ noinline int btrfs_cow_block(struct btrfs_trans_handle *trans,
 		btrfs_set_lock_blocking(parent);
 	btrfs_set_lock_blocking(buf);
 
+	/*
+	 * Before CoWing this block for later modification, check if it's
+	 * the subtree root and do the delayed subtree trace if needed.
+	 *
+	 * Also We don't care about the error, as it's handled internally.
+	 */
+	btrfs_qgroup_trace_subtree_after_cow(trans, root, buf);
 	ret = __btrfs_cow_block(trans, root, buf, parent,
 				 parent_slot, cow_ret, search_start, 0);
 

fs/btrfs/qgroup.c

@@ -3968,3 +3968,91 @@ out:
 			BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
 	return ret;
 }
+
+/*
+ * Check if the tree block is a subtree root, and if so do the needed
+ * delayed subtree trace for qgroup.
+ *
+ * This is called during btrfs_cow_block().
+ */
+int btrfs_qgroup_trace_subtree_after_cow(struct btrfs_trans_handle *trans,
+					 struct btrfs_root *root,
+					 struct extent_buffer *subvol_eb)
+{
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_qgroup_swapped_blocks *blocks = &root->swapped_blocks;
+	struct btrfs_qgroup_swapped_block *block;
+	struct extent_buffer *reloc_eb = NULL;
+	struct rb_node *node;
+	bool found = false;
+	bool swapped = false;
+	int level = btrfs_header_level(subvol_eb);
+	int ret = 0;
+	int i;
+
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
+		return 0;
+	if (!is_fstree(root->root_key.objectid) || !root->reloc_root)
+		return 0;
+
+	spin_lock(&blocks->lock);
+	if (!blocks->swapped) {
+		spin_unlock(&blocks->lock);
+		return 0;
+	}
+	node = blocks->blocks[level].rb_node;
+
+	while (node) {
+		block = rb_entry(node, struct btrfs_qgroup_swapped_block, node);
+		if (block->subvol_bytenr < subvol_eb->start) {
+			node = node->rb_left;
+		} else if (block->subvol_bytenr > subvol_eb->start) {
+			node = node->rb_right;
+		} else {
+			found = true;
+			break;
+		}
+	}
+	if (!found) {
+		spin_unlock(&blocks->lock);
+		goto out;
+	}
+	/* Found one, remove it from @blocks first and update blocks->swapped */
+	rb_erase(&block->node, &blocks->blocks[level]);
+	for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
+		if (RB_EMPTY_ROOT(&blocks->blocks[i])) {
+			swapped = true;
+			break;
+		}
+	}
+	blocks->swapped = swapped;
+	spin_unlock(&blocks->lock);
+
+	/* Read out reloc subtree root */
+	reloc_eb = read_tree_block(fs_info, block->reloc_bytenr,
+				   block->reloc_generation, block->level,
+				   &block->first_key);
+	if (IS_ERR(reloc_eb)) {
+		ret = PTR_ERR(reloc_eb);
+		reloc_eb = NULL;
+		goto free_out;
+	}
+	if (!extent_buffer_uptodate(reloc_eb)) {
+		ret = -EIO;
+		goto free_out;
+	}
+
+	ret = qgroup_trace_subtree_swap(trans, reloc_eb, subvol_eb,
+			block->last_snapshot, block->trace_leaf);
+free_out:
+	kfree(block);
+	free_extent_buffer(reloc_eb);
+out:
+	if (ret < 0) {
+		btrfs_err_rl(fs_info,
+			     "failed to account subtree at bytenr %llu: %d",
+			     subvol_eb->start, ret);
+		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+	}
+	return ret;
+}

fs/btrfs/qgroup.h

@@ -416,5 +416,7 @@ int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans,
 		struct extent_buffer *subvol_parent, int subvol_slot,
 		struct extent_buffer *reloc_parent, int reloc_slot,
 		u64 last_snapshot);
+int btrfs_qgroup_trace_subtree_after_cow(struct btrfs_trans_handle *trans,
+		struct btrfs_root *root, struct extent_buffer *eb);
 
 #endif

fs/btrfs/relocation.c

@@ -1889,16 +1889,12 @@ again:
 		 *    If not traced, we will leak data numbers
 		 * 2) Fs subtree
 		 *    If not traced, we will double count old data
-		 *    and tree block numbers, if current trans doesn't free
-		 *    data reloc tree inode.
+		 *
+		 * We don't scan the subtree right now, but only record
+		 * the swapped tree blocks.
+		 * The real subtree rescan is delayed until we have new
+		 * CoW on the subtree root node before transaction commit.
 		 */
-		ret = btrfs_qgroup_trace_subtree_swap(trans, rc->block_group,
-				parent, slot, path->nodes[level],
-				path->slots[level], last_snapshot);
-		if (ret < 0)
-			break;
-
-		btrfs_node_key_to_cpu(parent, &first_key, slot);
 		ret = btrfs_qgroup_add_swapped_blocks(trans, dest,
 				rc->block_group, parent, slot,
 				path->nodes[level], path->slots[level],