linux-kernel - [tip:core/locking] lockdep:reintroduce generation count to make BFS faster

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Date:	Wed, 22 Jul 2009 22:48:09 +0800
From:	tom.leiming@...il.com
To:	a.p.zijlstra@...llo.nl
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, torvalds@...ux-foundation.org, davem@...emloft.net,
	Ming Lei <tom.leiming@...il.com>
Subject: [tip:core/locking] lockdep:reintroduce generation count to make BFS faster

From: Ming Lei <tom.leiming@...il.com>

We still can apply DaveM's generation count optimization to
BFS, based on the following idea:

	.before doing each BFS, increase one to the global generation id;
	.if one node in the graph has been visited, marking it as visited
	by storing the current global generation id into the node's dep_gen_id field;
	.so we can decide if one node is visited by comparing the node's dep_gen_id
	with the global genration id.

By applying DaveM's genration count optimization to current implementation of BFS,
we can gains:
	.save MAX_LOCKDEP_ENTRIES/8 bytes memory;
	.remove the
		bitmap_zero(bfs_accessed, MAX_LOCKDEP_ENTRIES);
	in each BFS, which is very time-consuming since MAX_LOCKDEP_ENTRIES
        may be very large.(16384UL) 

Signed-off-by: Ming Lei <tom.leiming@...il.com>
---
 include/linux/lockdep.h |    1 +
 kernel/lockdep.c        |   11 ++++++-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 12aabfc..ddde26f 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -58,6 +58,7 @@ struct lock_class {
 
 	struct lockdep_subclass_key	*key;
 	unsigned int			subclass;
+	unsigned int			dep_gen_id;
 
 	/*
 	 * IRQ/softirq usage tracking bits:
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 1cedb00..1b1796a 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -848,14 +848,15 @@ struct circular_queue {
 };
 
 static struct circular_queue lock_cq;
-static unsigned long bfs_accessed[BITS_TO_LONGS(MAX_LOCKDEP_ENTRIES)];
 
 unsigned int max_bfs_queue_depth;
 
+static unsigned int lockdep_dependency_gen_id;
+
 static inline void __cq_init(struct circular_queue *cq)
 {
 	cq->front = cq->rear = 0;
-	bitmap_zero(bfs_accessed, MAX_LOCKDEP_ENTRIES);
+	lockdep_dependency_gen_id++;
 }
 
 static inline int __cq_empty(struct circular_queue *cq)
@@ -900,7 +901,7 @@ static inline void mark_lock_accessed(struct lock_list *lock,
 	nr = lock - list_entries;
 	WARN_ON(nr >= nr_list_entries);
 	lock->parent = parent;
-	set_bit(nr, bfs_accessed);
+	lock->class->dep_gen_id = lockdep_dependency_gen_id;
 }
 
 static inline unsigned long lock_accessed(struct lock_list *lock)
@@ -908,7 +909,7 @@ static inline unsigned long lock_accessed(struct lock_list *lock)
 	unsigned long nr;
 	nr = lock - list_entries;
 	WARN_ON(nr >= nr_list_entries);
-	return test_bit(nr, bfs_accessed);
+	return lock->class->dep_gen_id == lockdep_dependency_gen_id;
 }
 
 static inline struct lock_list *get_lock_parent(struct lock_list *child)
@@ -3491,7 +3492,7 @@ void __init lockdep_info(void)
 		sizeof(struct lock_chain) * MAX_LOCKDEP_CHAINS +
 		sizeof(struct list_head) * CHAINHASH_SIZE) / 1024
 #ifdef CONFIG_PROVE_LOCKING
-		+ sizeof(struct circular_queue) + sizeof(bfs_accessed)
+		+ sizeof(struct circular_queue)
 #endif
 		);
 
-- 
1.6.0.GIT

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