[RFC 1/2] mm, slub: introduce static key for slub_debug

[Vlastimil Babka]

One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be built
with it, but it's inactive until enabled on boot or at runtime, without
rebuilding the kernel. With a static key, we can minimize the overhead of
checking whether slub_debug is enabled, as we do for e.g. page_owner.

For now and for simplicity, the static key stays enabled for the whole uptime
once activated for any cache, although some per-cache debugging options can be
also disabled at runtime. This can be improved if there's interest.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/slub.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index d30ede89f4a6..398e53e16e2e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -115,10 +115,21 @@
  * 			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);
+	if (static_branch_unlikely(&slub_debug_enabled))
+		return s->flags & SLAB_DEBUG_FLAGS;
+	else
+		return 0;
 #else
 	return 0;
 #endif
@@ -1287,6 +1298,9 @@ static int __init setup_slub_debug(char *str)
 	if (*str == ',')
 		slub_debug_slabs = str + 1;
 out:
+	if (slub_debug)
+		static_branch_enable(&slub_debug_enabled);
+
 	return 1;
 }
 
@@ -5193,6 +5207,7 @@ static ssize_t red_zone_store(struct kmem_cache *s,
 	s->flags &= ~SLAB_RED_ZONE;
 	if (buf[0] == '1') {
 		s->flags |= SLAB_RED_ZONE;
+		static_branch_enable(&slub_debug_enabled);
 	}
 	calculate_sizes(s, -1);
 	return length;
@@ -5213,6 +5228,7 @@ static ssize_t poison_store(struct kmem_cache *s,
 	s->flags &= ~SLAB_POISON;
 	if (buf[0] == '1') {
 		s->flags |= SLAB_POISON;
+		static_branch_enable(&slub_debug_enabled);
 	}
 	calculate_sizes(s, -1);
 	return length;
@@ -5234,6 +5250,7 @@ static ssize_t store_user_store(struct kmem_cache *s,
 	if (buf[0] == '1') {
 		s->flags &= ~__CMPXCHG_DOUBLE;
 		s->flags |= SLAB_STORE_USER;
+		static_branch_enable(&slub_debug_enabled);
 	}
 	calculate_sizes(s, -1);
 	return length;
-- 
2.21.0