Commit 2aadf7fc authored Sep 08, 2017 by Davidlohr Bueso Committed by Linus Torvalds Sep 08, 2017

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>

parent cd9e61ed

lib/rbtree.c

+16 −7

Original line number	Diff line number	Diff line
		@@ -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 (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).
		*/
		if (!parent) {
		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);