slub: Add statistics for this_cmpxchg_double failures

Add some statistics for debugging.
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/linux/slub_def.h

@@ -32,6 +32,7 @@ enum stat_item {
 	DEACTIVATE_TO_TAIL,	/* Cpu slab was moved to the tail of partials */
 	DEACTIVATE_REMOTE_FREES,/* Slab contained remotely freed objects */
 	ORDER_FALLBACK,		/* Number of times fallback was necessary */
+	CMPXCHG_DOUBLE_CPU_FAIL,/* Failure of this_cpu_cmpxchg_double */
 	NR_SLUB_STAT_ITEMS };
 
 struct kmem_cache_cpu {

mm/slub.c

@@ -217,7 +217,7 @@ static inline void sysfs_slab_remove(struct kmem_cache *s)
 
 #endif
 
-static inline void stat(struct kmem_cache *s, enum stat_item si)
+static inline void stat(const struct kmem_cache *s, enum stat_item si)
 {
 #ifdef CONFIG_SLUB_STATS
 	__this_cpu_inc(s->cpu_slab->stat[si]);
@@ -1597,6 +1597,7 @@ static inline void note_cmpxchg_failure(const char *n,
 		printk("for unknown reason: actual=%lx was=%lx target=%lx\n",
 			actual_tid, tid, next_tid(tid));
 #endif
+	stat(s, CMPXCHG_DOUBLE_CPU_FAIL);
 }
 
 #endif