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

tom.leiming
2009-07-22T14:48:20Z

From: Ming Lei <tom.leiming@gmail.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@gmail.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