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From: Davidlohr Bueso <dave@stgolabs.net>
To: akpm@linux-foundation.org
Cc: mingo@kernel.org, peterz@infradead.org, jack@suse.cz,
	torvalds@linux-foundation.org, kirill.shutemov@linux.intel.com,
	hch@infradead.org, ldufour@linux.vnet.ibm.com, mhocko@suse.com,
	mgorman@techsingularity.net, dave@stgolabs.net,
	linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: [PATCH 02/17] rbtree: optimize root-check during rebalancing loop
Date: Tue, 18 Jul 2017 18:45:48 -0700
Message-ID: <20170719014603.19029-3-dave@stgolabs.net> (raw)
In-Reply-To: <20170719014603.19029-1-dave@stgolabs.net>

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

  parent reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-19  1:45 [PATCH -next v4 00/17] rbtree: cache leftmost node internally Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 01/17] " Davidlohr Bueso
2017-07-19  1:45 ` Davidlohr Bueso [this message]
2017-07-19  1:45 ` [PATCH 03/17] rbtree: add some additional comments for rebalancing cases Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 04/17] lib/rbtree_test.c: make input module parameters Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 05/17] lib/rbtree_test.c: add (inorder) traversal test Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 06/17] lib/rbtree_test.c: support rb_root_cached Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 07/17] sched/fair: replace cfs_rq->rb_leftmost Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 08/17] sched/deadline: replace earliest dl and rq leftmost caching Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 09/17] locking/rtmutex: replace top-waiter and pi_waiters " Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 10/17] block/cfq: replace cfq_rb_root " Davidlohr Bueso
2017-07-19  7:46   ` Jan Kara
2017-07-19  1:45 ` [PATCH 11/17] lib/interval_tree: fast overlap detection Davidlohr Bueso
2017-07-22 17:52   ` Doug Ledford
2017-08-01 17:16   ` Michael S. Tsirkin
2017-07-19  1:45 ` [PATCH 12/17] lib/interval-tree: correct comment wrt generic flavor Davidlohr Bueso
2017-07-19  1:45 ` [PATCH 13/17] procfs: use faster rb_first_cached() Davidlohr Bueso
2017-07-19  1:46 ` [PATCH 14/17] fs/epoll: " Davidlohr Bueso
2017-07-19  1:46 ` [PATCH 15/17] fs/ext4: use cached rbtrees Davidlohr Bueso
2017-07-19  7:40   ` Jan Kara
2017-07-19 22:50     ` Davidlohr Bueso
2017-07-19  1:46 ` [PATCH 16/17] mem/memcg: cache rightmost node Davidlohr Bueso
2017-07-19  7:50   ` Michal Hocko
2017-07-26 21:09     ` Andrew Morton
2017-07-27  7:06       ` Michal Hocko
2017-07-19  1:46 ` [PATCH 17/17] block/cfq: cache rightmost rb_node Davidlohr Bueso
2017-07-19  7:59   ` Jan Kara

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