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

tip-bot for Ming Lei
2009-08-02T13:15:42Z

Commit-ID: 67a8b0af3942250e563739442f98d3bca50442ff
Gitweb: http://git.kernel.org/tip/67a8b0af3942250e563739442f98d3bca50442ff
Author: Ming Lei <tom.leiming@gmail.com>
AuthorDate: Wed, 22 Jul 2009 22:48:09 +0800
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Sun, 2 Aug 2009 14:59:32 +0200

lockdep: Reintroduce generation count to make BFS faster

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

- before doing each BFS, increase the global generation id
by 1

- if one node in the graph has been visited, mark 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 has been visited already, by
comparing the node's dep_gen_id with the global generation id.

By applying DaveM's generation count optimization to current
implementation of BFS, we gain the following advantages:

- we save MAX_LOCKDEP_ENTRIES/8 bytes memory;

- we 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>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "David S. Miller" <davem@davemloft.net>
LKML-Reference: <1248274089-6358-1-git-send-email-tom.leiming@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
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 47d42ef..9ccf0e2 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 0bb246e..2b443b5 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -861,14 +861,15 @@ 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)
@@ -914,7 +915,7 @@ 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)
@@ -923,7 +924,7 @@ static inline unsigned long lock_accessed(struct lock_list *lock)

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)
@@ -3604,7 +3605,7 @@ void __init lockdep_info(void)