[PATCH 6/9] mm, slub: introduce static key for slub_debug()

[Vlastimil Babka <vbabka@suse.cz>]

One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be built
with the option enabled, but it's inactive until simply enabled on boot,
without rebuilding the kernel. With a static key, we can further eliminate the
overhead of checking whether a cache has a particular debug flag enabled if we
know that there are no such caches (slub_debug was not enabled during boot). We
use the same mechanism also for e.g.  page_owner, debug_pagealloc or kmemcg
functionality.

This patch introduces the static key and makes the general check for per-cache
debug flags kmem_cache_debug() use it. This benefits several call sites,
including (slow path but still rather frequent) __slab_free(). The next patches
will add more uses.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8dd2925448ae..24d3e5f832aa 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -114,13 +114,21 @@
  * 			the fast path and disables lockless freelists.
  */
 
+#ifdef CONFIG_SLUB_DEBUG
+#ifdef CONFIG_SLUB_DEBUG_ON
+DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
+#else
+DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
+#endif
+#endif
+
 static inline int kmem_cache_debug(struct kmem_cache *s)
 {
 #ifdef CONFIG_SLUB_DEBUG
-	return unlikely(s->flags & SLAB_DEBUG_FLAGS);
-#else
-	return 0;
+	if (static_branch_unlikely(&slub_debug_enabled))
+		return s->flags & SLAB_DEBUG_FLAGS;
 #endif
+	return 0;
 }
 
 void *fixup_red_left(struct kmem_cache *s, void *p)
@@ -1389,6 +1397,8 @@ static int __init setup_slub_debug(char *str)
 		slub_debug_string = saved_str;
 	}
 out:
+	if (slub_debug != 0 || slub_debug_string)
+		static_branch_enable(&slub_debug_enabled);
 	if ((static_branch_unlikely(&init_on_alloc) ||
 	     static_branch_unlikely(&init_on_free)) &&
 	    (slub_debug & SLAB_POISON))
-- 
2.26.2