[PATCH 2/2] locking/lockdep: Optimize look_up_lock_class()

[Waiman Long <longman@redhat.com>]

The lockdep_map structure has a built-in cache for the first two
subclasses.  However, when lockdep_init_map_type() is called with a
subclass other than 0, it forces class_cache[0] to hold class for a
non-0 subclass. That messes up the utility of the cache as you never
know if class_cache[0] holds a class for subclass 0.

Use the 1-byte hole in the lockdep_map structure to hold the
subclass information in class_cache[0] so that we can reliably use the
class_cache[] information to get the class based on the given subclass
quicker in look_up_lock_class().

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/lockdep_types.h |  2 +-
 kernel/locking/lockdep.c      | 19 +++++++++++++++++--
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
index 9c62d274b71e..2c2a19a0b645 100644
--- a/include/linux/lockdep_types.h
+++ b/include/linux/lockdep_types.h
@@ -176,7 +176,7 @@ struct lockdep_map {
 	u8				wait_type_outer; /* can be taken in this context */
 	u8				wait_type_inner; /* presents this context */
 	u8				lock_type;
-	/* u8				hole; */
+	u8				cache0_subclass; /* class_cache[0] subclass */
 #ifdef CONFIG_LOCK_STAT
 	int				cpu;
 	unsigned long			ip;
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 4a882f83aeb9..0b19d153ffa4 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -881,6 +881,18 @@ look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass)
 	if (unlikely(!lock->key))
 		return NULL;
 
+	/*
+	 * Use the class_cache[] to get the class information if available.
+	 */
+	if (unlikely(lock->cache0_subclass)) {
+		if (subclass == lock->cache0_subclass)
+			return lock->class_cache[0];
+	} else if (likely(subclass < NR_LOCKDEP_CACHING_CLASSES)) {
+		class = lock->class_cache[subclass];
+		if (class)
+			return class;
+	}
+
 	/*
 	 * NOTE: the class-key must be unique. For dynamic locks, a static
 	 * lock_class_key variable is passed in through the mutex_init()
@@ -1335,10 +1347,12 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 	graph_unlock();
 
 out_set_class_cache:
-	if (!subclass || force)
+	if (subclass && force) {
 		lock->class_cache[0] = class;
-	else if (subclass < NR_LOCKDEP_CACHING_CLASSES)
+		lock->cache0_subclass = subclass;
+	} else if (subclass < NR_LOCKDEP_CACHING_CLASSES) {
 		lock->class_cache[subclass] = class;
+	}
 
 	/*
 	 * Hash collision, did we smoke some? We found a class with a matching
@@ -4797,6 +4811,7 @@ void lockdep_init_map_type(struct lockdep_map *lock, const char *name,
 	lock->wait_type_outer = outer;
 	lock->wait_type_inner = inner;
 	lock->lock_type = lock_type;
+	lock->cache0_subclass = 0;
 
 	/*
 	 * No key, no joy, we need to hash something.
-- 
2.27.0