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

Hyeonggon Yoo
2023-12-07T02:40:15Z

On Mon, Nov 20, 2023 at 07:34:32PM +0100, Vlastimil Babka wrote:
> 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);
> }
>
>
> --

Looks good to me,
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

> 2.42.1
>
>