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Subject[PATCH 02/17] rbtree: optimize root-check during rebalancing loop
Date
The only times the nil-parent (root node) condition is true is
when the node is the first in the tree, or after fixing rbtree
rule #4 and the case 1 rebalancing made the node the root. Such
conditions do not apply most of the time:

(i) The common case in an rbtree is to have more than a single
node, so this is only true for the first rb_insert().

(ii) While there is a chance only one first rotation is needed,
cases where the node's uncle is black (cases 2,3) are more
common as we can have the following scenarios during the rotation
looping:

case1 only, case1+1, case2+3, case1+2+3, case3 only, etc.

This patch, therefore, adds an unlikely() optimization to this
conditional. When profiling with CONFIG_PROFILE_ANNOTATED_BRANCHES,
a kernel build shows that the incorrect rate is less than 15%, and
for workloads that involve insert mostly trees overtime tend to
have less than 2% incorrect rate.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
lib/rbtree.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/lib/rbtree.c b/lib/rbtree.c
index d102d9d2ffaa..e7cce12f404f 100644
--- a/lib/rbtree.c
+++ b/lib/rbtree.c
@@ -105,16 +105,25 @@ __rb_insert(struct rb_node *node, struct rb_root *root,

while (true) {
/*
- * Loop invariant: node is red
- *
- * If there is a black parent, we are done.
- * Otherwise, take some corrective action as we don't
- * want a red root or two consecutive red nodes.
+ * Loop invariant: node is red.
*/
- if (!parent) {
+ if (unlikely(!parent)) {
+ /*
+ * The inserted node is root. Either this is the
+ * first node, or we recursed at Case 1 below and
+ * are no longer violating 4).
+ */
rb_set_parent_color(node, NULL, RB_BLACK);
break;
- } else if (rb_is_black(parent))
+ }
+
+ /*
+ * If there is a black parent, we are done.
+ * Otherwise, take some corrective action as,
+ * per 4), we don't want a red root or two
+ * consecutive red nodes.
+ */
+ if(rb_is_black(parent))
break;

gparent = rb_red_parent(parent);
--
2.12.0
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