[PATCH 2/2] lockdep: always try to set ->class_cache inregister_lock_class() lockdep_init_map()

[Yong Zhang]

From: Yong Zhang <yong.zhang0@xxxxxxxxx>

commit [62016250 lockdep: Add improved subclass caching] try to
improve performance (expecially to reduce the cost of rq->lock)
when using lockdep, but it fails due to lockdep_init_map() in
which ->class_cache is cleared. The typical caller is
lock_set_subclass(), after that class will not be cached anymore.

This patch try to achive the goal of commit 62016250 by always
setting ->class_cache in register_lock_class().

=== Score comparison of benchmarks ===

for i in `seq 1 10`; do ./perf bench -f simple sched messaging; done

before:  min: 0.604, max: 0.660, avg: 0.622
after:   min: 0.414, max: 0.473, avg: 0.427

for i in `seq 1 10`; do ./perf bench -f simple sched messaging -g 40; done

before:  min: 2.347, max: 2.421, avg: 2.391
after:   min: 1.652, max: 1.699, avg: 1.671

Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
Cc: Hitoshi Mitake <mitake@xxxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/lockdep.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 21ea1dc..e9cbb99 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -722,7 +722,7 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
 
 	class = look_up_lock_class(lock, subclass);
 	if (likely(class))
-		return class;
+		goto out_set_class_cache;
 
 	/*
 	 * Debug-check: all keys must be persistent!
@@ -807,6 +807,7 @@ out_unlock_set:
 	graph_unlock();
 	raw_local_irq_restore(flags);
 
+out_set_class_cache:
 	if (!subclass || force)
 		lock->class_cache[0] = class;
 	else if (subclass < NR_LOCKDEP_CACHING_CLASSES)
-- 
1.7.5.4

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