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authorDavidlohr Bueso <dave@stgolabs.net>2017-09-08 16:14:39 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-08 18:26:48 -0700
commit2aadf7fc7df9e70c99786ffb8452ccdd83d49e59 (patch)
tree91b8d6c896cb417ea43470e3413be515a7bcf22d
parentcd9e61ed1eebbcd5dfad59475d41ec58d9b64b6a (diff)
rbtree: optimize root-check during rebalancing loop
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. Link: http://lkml.kernel.org/r/20170719014603.19029-3-dave@stgolabs.net Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--lib/rbtree.c23
1 files 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);