[PATCH v2 21/21] mm/slub: optimize free fast path code layout

Vlastimil Babka
2023-11-20T18:34:46Z

Inspection of kmem_cache_free() disassembly showed we could make the
fast path smaller by providing few more hints to the compiler, and
splitting the memcg_slab_free_hook() into an inline part that only
checks if there's work to do, and an out of line part doing the actual
uncharge.

bloat-o-meter results:
add/remove: 2/0 grow/shrink: 0/3 up/down: 286/-554 (-268)
Function old new delta
__memcg_slab_free_hook - 270 +270
__pfx___memcg_slab_free_hook - 16 +16
kfree 828 665 -163
kmem_cache_free 1116 948 -168
kmem_cache_free_bulk.part 1701 1478 -223

Checking kmem_cache_free() disassembly now shows the non-fastpath
cases are handled out of line, which should reduce instruction cache
usage.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/slub.c | 40 ++++++++++++++++++++++++----------------
1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 77d259f3d592..3f8b95757106 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1959,20 +1959,11 @@ void memcg_slab_post_alloc_hook(struct kmem_cache *s, struct obj_cgroup *objcg,
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)
+static void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
+ void **p, int objects,
+ struct obj_cgroup **objcgs)
{
- struct obj_cgroup **objcgs;
- int i;
-
- if (!memcg_kmem_online())
- return;
-
- objcgs = slab_objcgs(slab);
- if (!objcgs)
- return;
-
- for (i = 0; i < objects; i++) {
+ for (int i = 0; i < objects; i++) {
struct obj_cgroup *objcg;
unsigned int off;

@@ -1988,6 +1979,22 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
obj_cgroup_put(objcg);
}
}
+
+static __fastpath_inline
+void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
+ int objects)
+{
+ struct obj_cgroup **objcgs;
+
+ if (!memcg_kmem_online())
+ return;
+
+ objcgs = slab_objcgs(slab);
+ if (likely(!objcgs))
+ return;
+
+ __memcg_slab_free_hook(s, slab, p, objects, objcgs);
+}
#else /* CONFIG_MEMCG_KMEM */
static inline struct mem_cgroup *memcg_from_slab_obj(void *ptr)
{
@@ -2047,7 +2054,7 @@ static __always_inline bool slab_free_hook(struct kmem_cache *s,
* The initialization memset's clear the object and the metadata,
* but don't touch the SLAB redzone.
*/
- if (init) {
+ if (unlikely(init)) {
int rsize;

if (!kasan_has_integrated_init())
@@ -2083,7 +2090,8 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
next = get_freepointer(s, object);

/* If object's reuse doesn't have to be delayed */
- if (!slab_free_hook(s, object, slab_want_init_on_free(s))) {
+ if (likely(!slab_free_hook(s, object,
+ slab_want_init_on_free(s)))) {
/* Move object to the new freelist */
set_freepointer(s, object, *head);
*head = object;
@@ -4282,7 +4290,7 @@ static __fastpath_inline void slab_free(struct kmem_cache *s, struct slab *slab,
* With KASAN enabled slab_free_freelist_hook modifies the freelist
* to remove objects, whose reuse must be delayed.
*/
- if (slab_free_freelist_hook(s, &head, &tail, &cnt))
+ if (likely(slab_free_freelist_hook(s, &head, &tail, &cnt)))
do_slab_free(s, slab, head, tail, cnt, addr);
}

--
2.42.1