[PATCH v2 20/21] mm/slub: optimize alloc fastpath code layout

[Vlastimil Babka <vbabka@suse.cz>]

With allocation fastpaths no longer divided between two .c files, we
have better inlining, however checking the disassembly of
kmem_cache_alloc() reveals we can do better to make the fastpaths
smaller and move the less common situations out of line or to separate
functions, to reduce instruction cache pressure.

- split memcg pre/post alloc hooks to inlined checks that use likely()
  to assume there will be no objcg handling necessary, and non-inline
  functions doing the actual handling

- add some more likely/unlikely() to pre/post alloc hooks to indicate
  which scenarios should be out of line

- change gfp_allowed_mask handling in slab_post_alloc_hook() so the
  code can be optimized away when kasan/kmsan/kmemleak is configured out

bloat-o-meter shows:
add/remove: 4/2 grow/shrink: 1/8 up/down: 521/-2924 (-2403)
Function                                     old     new   delta
__memcg_slab_post_alloc_hook                   -     461    +461
kmem_cache_alloc_bulk                        775     791     +16
__pfx_should_failslab.constprop                -      16     +16
__pfx___memcg_slab_post_alloc_hook             -      16     +16
should_failslab.constprop                      -      12     +12
__pfx_memcg_slab_post_alloc_hook              16       -     -16
kmem_cache_alloc_lru                        1295    1023    -272
kmem_cache_alloc_node                       1118     817    -301
kmem_cache_alloc                            1076     772    -304
kmalloc_node_trace                          1149     838    -311
kmalloc_trace                               1102     789    -313
__kmalloc_node_track_caller                 1393    1080    -313
__kmalloc_node                              1397    1082    -315
__kmalloc                                   1374    1059    -315
memcg_slab_post_alloc_hook                   464       -    -464

Note that gcc still decided to inline __memcg_pre_alloc_hook(), but the
code is out of line. Forcing noinline did not improve the results. As a
result the fastpaths are shorter and overal code size is reduced.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 89 ++++++++++++++++++++++++++++++++++++++-------------------------
 1 file changed, 54 insertions(+), 35 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 5683f1d02e4f..77d259f3d592 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1866,25 +1866,17 @@ static inline size_t obj_full_size(struct kmem_cache *s)
 /*
  * Returns false if the allocation should fail.
  */
-static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
-					     struct list_lru *lru,
-					     struct obj_cgroup **objcgp,
-					     size_t objects, gfp_t flags)
+static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s,
+					struct list_lru *lru,
+					struct obj_cgroup **objcgp,
+					size_t objects, gfp_t flags)
 {
-	struct obj_cgroup *objcg;
-
-	if (!memcg_kmem_online())
-		return true;
-
-	if (!(flags & __GFP_ACCOUNT) && !(s->flags & SLAB_ACCOUNT))
-		return true;
-
 	/*
 	 * The obtained objcg pointer is safe to use within the current scope,
 	 * defined by current task or set_active_memcg() pair.
 	 * obj_cgroup_get() is used to get a permanent reference.
 	 */
-	objcg = current_obj_cgroup();
+	struct obj_cgroup *objcg = current_obj_cgroup();
 	if (!objcg)
 		return true;
 
@@ -1907,17 +1899,34 @@ static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
 	return true;
 }
 
-static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
-					      struct obj_cgroup *objcg,
-					      gfp_t flags, size_t size,
-					      void **p)
+/*
+ * Returns false if the allocation should fail.
+ */
+static __fastpath_inline
+bool memcg_slab_pre_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
+			       struct obj_cgroup **objcgp, size_t objects,
+			       gfp_t flags)
+{
+	if (!memcg_kmem_online())
+		return true;
+
+	if (likely(!(flags & __GFP_ACCOUNT) && !(s->flags & SLAB_ACCOUNT)))
+		return true;
+
+	return likely(__memcg_slab_pre_alloc_hook(s, lru, objcgp, objects,
+						  flags));
+}
+
+static void __memcg_slab_post_alloc_hook(struct kmem_cache *s,
+					 struct obj_cgroup *objcg,
+					 gfp_t flags, size_t size,
+					 void **p)
 {
 	struct slab *slab;
 	unsigned long off;
 	size_t i;
 
-	if (!memcg_kmem_online() || !objcg)
-		return;
+	flags &= gfp_allowed_mask;
 
 	for (i = 0; i < size; i++) {
 		if (likely(p[i])) {
@@ -1940,6 +1949,16 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 	}
 }
 
+static __fastpath_inline
+void memcg_slab_post_alloc_hook(struct kmem_cache *s, struct obj_cgroup *objcg,
+				gfp_t flags, size_t size, void **p)
+{
+	if (likely(!memcg_kmem_online() || !objcg))
+		return;
+
+	return __memcg_slab_post_alloc_hook(s, objcg, flags, size, p);
+}
+
 static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
 					void **p, int objects)
 {
@@ -3709,34 +3728,34 @@ noinline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
 }
 ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
 
-static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
-						     struct list_lru *lru,
-						     struct obj_cgroup **objcgp,
-						     size_t size, gfp_t flags)
+static __fastpath_inline
+struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
+				       struct list_lru *lru,
+				       struct obj_cgroup **objcgp,
+				       size_t size, gfp_t flags)
 {
 	flags &= gfp_allowed_mask;
 
 	might_alloc(flags);
 
-	if (should_failslab(s, flags))
+	if (unlikely(should_failslab(s, flags)))
 		return NULL;
 
-	if (!memcg_slab_pre_alloc_hook(s, lru, objcgp, size, flags))
+	if (unlikely(!memcg_slab_pre_alloc_hook(s, lru, objcgp, size, flags)))
 		return NULL;
 
 	return s;
 }
 
-static inline void slab_post_alloc_hook(struct kmem_cache *s,
-					struct obj_cgroup *objcg, gfp_t flags,
-					size_t size, void **p, bool init,
-					unsigned int orig_size)
+static __fastpath_inline
+void slab_post_alloc_hook(struct kmem_cache *s,	struct obj_cgroup *objcg,
+			  gfp_t flags, size_t size, void **p, bool init,
+			  unsigned int orig_size)
 {
 	unsigned int zero_size = s->object_size;
 	bool kasan_init = init;
 	size_t i;
-
-	flags &= gfp_allowed_mask;
+	gfp_t init_flags = flags & gfp_allowed_mask;
 
 	/*
 	 * For kmalloc object, the allocated memory size(object_size) is likely
@@ -3769,13 +3788,13 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s,
 	 * As p[i] might get tagged, memset and kmemleak hook come after KASAN.
 	 */
 	for (i = 0; i < size; i++) {
-		p[i] = kasan_slab_alloc(s, p[i], flags, kasan_init);
+		p[i] = kasan_slab_alloc(s, p[i], init_flags, kasan_init);
 		if (p[i] && init && (!kasan_init ||
 				     !kasan_has_integrated_init()))
 			memset(p[i], 0, zero_size);
 		kmemleak_alloc_recursive(p[i], s->object_size, 1,
-					 s->flags, flags);
-		kmsan_slab_alloc(s, p[i], flags);
+					 s->flags, init_flags);
+		kmsan_slab_alloc(s, p[i], init_flags);
 	}
 
 	memcg_slab_post_alloc_hook(s, objcg, flags, size, p);
@@ -3799,7 +3818,7 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
 	bool init = false;
 
 	s = slab_pre_alloc_hook(s, lru, &objcg, 1, gfpflags);
-	if (!s)
+	if (unlikely(!s))
 		return NULL;
 
 	object = kfence_alloc(s, orig_size, gfpflags);

-- 
2.42.1