Btrfs: delayed-refs: use rb_first_cached for href_root

rb_first_cached() trades an extra pointer "leftmost" for doing the same
job as rb_first() but in O(1).

Functions manipulating href_root need to get the first entry, this
converts href_root to use rb_first_cached().

This patch is first in the sequenct of similar updates to other rbtrees
and this is analysis of the expected behaviour and improvements.

There's a common pattern:

while (node = rb_first) {
        entry = rb_entry(node)
        next = rb_next(node)
        rb_erase(node)
        cleanup(entry)
}

rb_first needs to traverse the tree up to logN depth, rb_erase can
completely reshuffle the tree. With the caching we'll skip the traversal
in rb_first.  That's a cached memory access vs looped pointer
dereference trade-off that IMHO has a clear winner.

Measurements show there's not much difference in a sample tree with
10000 nodes: 4.5s / rb_first and 4.8s / rb_first_cached. Real effects of
caching and pointer chasing are unpredictable though.

Further optimzations can be done to avoid the expensive rb_erase step.
In some cases it's ok to process the nodes in any order, so the tree can
be traversed in post-order, not rebalancing the children nodes and just
calling free. Care must be taken regarding the next node.

Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog from mail discussions ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-ref.c

@@ -101,14 +101,15 @@ static int comp_refs(struct btrfs_delayed_ref_node *ref1,
 }
 
 /* insert a new ref to head ref rbtree */
-static struct btrfs_delayed_ref_head *htree_insert(struct rb_root *root,
+static struct btrfs_delayed_ref_head *htree_insert(struct rb_root_cached *root,
 						   struct rb_node *node)
 {
-	struct rb_node **p = &root->rb_node;
+	struct rb_node **p = &root->rb_root.rb_node;
 	struct rb_node *parent_node = NULL;
 	struct btrfs_delayed_ref_head *entry;
 	struct btrfs_delayed_ref_head *ins;
 	u64 bytenr;
+	bool leftmost = true;
 
 	ins = rb_entry(node, struct btrfs_delayed_ref_head, href_node);
 	bytenr = ins->bytenr;
@@ -117,16 +118,18 @@ static struct btrfs_delayed_ref_head *htree_insert(struct rb_root *root,
 		entry = rb_entry(parent_node, struct btrfs_delayed_ref_head,
 				 href_node);
 
-		if (bytenr < entry->bytenr)
+		if (bytenr < entry->bytenr) {
 			p = &(*p)->rb_left;
-		else if (bytenr > entry->bytenr)
+		} else if (bytenr > entry->bytenr) {
 			p = &(*p)->rb_right;
-		else
+			leftmost = false;
+		} else {
 			return entry;
 		}
+	}
 
 	rb_link_node(node, parent_node, p);
-	rb_insert_color(node, root);
+	rb_insert_color_cached(node, root, leftmost);
 	return NULL;
 }
 
@@ -164,10 +167,11 @@ static struct btrfs_delayed_ref_node* tree_insert(struct rb_root *root,
  * If return_bigger is given, the next bigger entry is returned if no exact
  * match is found.
  */
-static struct btrfs_delayed_ref_head *
-find_ref_head(struct rb_root *root, u64 bytenr,
+static struct btrfs_delayed_ref_head* find_ref_head(
+		struct btrfs_delayed_ref_root *dr, u64 bytenr,
 		int return_bigger)
 {
+	struct rb_root *root = &dr->href_root.rb_root;
 	struct rb_node *n;
 	struct btrfs_delayed_ref_head *entry;
 
@@ -187,7 +191,7 @@ find_ref_head(struct rb_root *root, u64 bytenr,
 		if (bytenr > entry->bytenr) {
 			n = rb_next(&entry->href_node);
 			if (!n)
-				n = rb_first(root);
+				n = rb_first_cached(&dr->href_root);
 			entry = rb_entry(n, struct btrfs_delayed_ref_head,
 					 href_node);
 			return entry;
@@ -357,12 +361,12 @@ btrfs_select_ref_head(struct btrfs_trans_handle *trans)
 
 again:
 	start = delayed_refs->run_delayed_start;
-	head = find_ref_head(&delayed_refs->href_root, start, 1);
+	head = find_ref_head(delayed_refs, start, 1);
 	if (!head && !loop) {
 		delayed_refs->run_delayed_start = 0;
 		start = 0;
 		loop = true;
-		head = find_ref_head(&delayed_refs->href_root, start, 1);
+		head = find_ref_head(delayed_refs, start, 1);
 		if (!head)
 			return NULL;
 	} else if (!head && loop) {
@@ -903,7 +907,7 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
 struct btrfs_delayed_ref_head *
 btrfs_find_delayed_ref_head(struct btrfs_delayed_ref_root *delayed_refs, u64 bytenr)
 {
-	return find_ref_head(&delayed_refs->href_root, bytenr, 0);
+	return find_ref_head(delayed_refs, bytenr, 0);
 }
 
 void __cold btrfs_delayed_ref_exit(void)

fs/btrfs/delayed-ref.h

@@ -148,7 +148,7 @@ struct btrfs_delayed_data_ref {
 
 struct btrfs_delayed_ref_root {
 	/* head ref rbtree */
-	struct rb_root href_root;
+	struct rb_root_cached href_root;
 
 	/* dirty extent records */
 	struct rb_root dirty_extent_root;

fs/btrfs/disk-io.c

@@ -4203,7 +4203,7 @@ static int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
 		return ret;
 	}
 
-	while ((node = rb_first(&delayed_refs->href_root)) != NULL) {
+	while ((node = rb_first_cached(&delayed_refs->href_root)) != NULL) {
 		struct btrfs_delayed_ref_head *head;
 		struct rb_node *n;
 		bool pin_bytes = false;
@@ -4239,7 +4239,7 @@ static int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
 		if (head->processing == 0)
 			delayed_refs->num_heads_ready--;
 		atomic_dec(&delayed_refs->num_entries);
-		rb_erase(&head->href_node, &delayed_refs->href_root);
+		rb_erase_cached(&head->href_node, &delayed_refs->href_root);
 		RB_CLEAR_NODE(&head->href_node);
 		spin_unlock(&head->lock);
 		spin_unlock(&delayed_refs->lock);

fs/btrfs/extent-tree.c

@@ -2454,7 +2454,7 @@ static int cleanup_ref_head(struct btrfs_trans_handle *trans,
 		return 1;
 	}
 	delayed_refs->num_heads--;
-	rb_erase(&head->href_node, &delayed_refs->href_root);
+	rb_erase_cached(&head->href_node, &delayed_refs->href_root);
 	RB_CLEAR_NODE(&head->href_node);
 	spin_unlock(&head->lock);
 	spin_unlock(&delayed_refs->lock);
@@ -2940,7 +2940,7 @@ again:
 			btrfs_create_pending_block_groups(trans);
 
 		spin_lock(&delayed_refs->lock);
-		node = rb_first(&delayed_refs->href_root);
+		node = rb_first_cached(&delayed_refs->href_root);
 		if (!node) {
 			spin_unlock(&delayed_refs->lock);
 			goto out;
@@ -6929,7 +6929,7 @@ static noinline int check_ref_cleanup(struct btrfs_trans_handle *trans,
 	 * at this point we have a head with no other entries.  Go
 	 * ahead and process it.
 	 */
-	rb_erase(&head->href_node, &delayed_refs->href_root);
+	rb_erase_cached(&head->href_node, &delayed_refs->href_root);
 	RB_CLEAR_NODE(&head->href_node);
 	atomic_dec(&delayed_refs->num_entries);
 

fs/btrfs/transaction.c

@@ -44,7 +44,8 @@ void btrfs_put_transaction(struct btrfs_transaction *transaction)
 	WARN_ON(refcount_read(&transaction->use_count) == 0);
 	if (refcount_dec_and_test(&transaction->use_count)) {
 		BUG_ON(!list_empty(&transaction->list));
-		WARN_ON(!RB_EMPTY_ROOT(&transaction->delayed_refs.href_root));
+		WARN_ON(!RB_EMPTY_ROOT(
+				&transaction->delayed_refs.href_root.rb_root));
 		if (transaction->delayed_refs.pending_csums)
 			btrfs_err(transaction->fs_info,
 				  "pending csums is %llu",
@@ -245,7 +246,7 @@ loop:
 
 	memset(&cur_trans->delayed_refs, 0, sizeof(cur_trans->delayed_refs));
 
-	cur_trans->delayed_refs.href_root = RB_ROOT;
+	cur_trans->delayed_refs.href_root = RB_ROOT_CACHED;
 	cur_trans->delayed_refs.dirty_extent_root = RB_ROOT;
 	atomic_set(&cur_trans->delayed_refs.num_entries, 0);