[PATCH 2/3] alloc_tag: uninline code gated by mem_alloc_profiling_key in slab allocator

Suren Baghdasaryan
2025-01-26T07:02:12Z

When a sizable code section is protected by a disabled static key, that
code gets into the instruction cache even though it's not executed and
consumes the cache, increasing cache misses. This can be remedied by
moving such code into a separate uninlined function. The improvement
however comes at the expense of the configuration when this static key
gets enabled since there is now an additional function call.
The default state of the mem_alloc_profiling_key is controlled by
CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT. Apply this optimization
only if CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n, improving the
performance of the default configuration.
When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y the functions
are inlined and performance does not change.

On a Pixel6 phone, slab allocation profiling overhead measured with
CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n and profiling disabled:

baseline modified
Big 3.31% 0.17%
Medium 3.79% 0.57%
Little 6.68% 1.28%

When CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=n and memory allocation
profiling gets enabled, the difference in performance before and after
this change stays within noise levels.

On x86 this patch does not make noticeable difference because the overhead
with mem_alloc_profiling_key disabled is much lower (under 1%) to start
with, so any improvement is less visible and hard to distinguish from the
noise.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
include/linux/alloc_tag.h | 6 +++++
mm/slub.c | 46 ++++++++++++++++++++++++---------------
2 files changed, 34 insertions(+), 18 deletions(-)

diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index a946e0203e6d..c5de2a0c1780 100644
--- a/include/linux/alloc_tag.h
+++ b/include/linux/alloc_tag.h
@@ -116,6 +116,12 @@ DECLARE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
DECLARE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
mem_alloc_profiling_key);

+#ifdef CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
+#define inline_if_mem_alloc_prof inline
+#else
+#define inline_if_mem_alloc_prof noinline
+#endif
+
static inline bool mem_alloc_profiling_enabled(void)
{
return static_branch_maybe(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
diff --git a/mm/slub.c b/mm/slub.c
index 996691c137eb..3107d43dfddc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2000,7 +2000,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
return 0;
}

-static inline void free_slab_obj_exts(struct slab *slab)
+static inline_if_mem_alloc_prof void free_slab_obj_exts(struct slab *slab)
{
struct slabobj_ext *obj_exts;

@@ -2077,33 +2077,35 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
return slab_obj_exts(slab) + obj_to_index(s, slab, p);
}

-static inline void
-alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
+static inline_if_mem_alloc_prof void
+__alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
{
- if (need_slab_obj_ext()) {
- struct slabobj_ext *obj_exts;
+ struct slabobj_ext *obj_exts;

- obj_exts = prepare_slab_obj_exts_hook(s, flags, object);
- /*
- * Currently obj_exts is used only for allocation profiling.
- * If other users appear then mem_alloc_profiling_enabled()
- * check should be added before alloc_tag_add().
- */
- if (likely(obj_exts))
- alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
- }
+ obj_exts = prepare_slab_obj_exts_hook(s, flags, object);
+ /*
+ * Currently obj_exts is used only for allocation profiling.
+ * If other users appear then mem_alloc_profiling_enabled()
+ * check should be added before alloc_tag_add().
+ */
+ if (likely(obj_exts))
+ alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
}

static inline void
-alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
+alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
+{
+ if (need_slab_obj_ext())
+ __alloc_tagging_slab_alloc_hook(s, object, flags);
+}
+
+static inline_if_mem_alloc_prof void
+__alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
int objects)
{
struct slabobj_ext *obj_exts;
int i;

- if (!mem_alloc_profiling_enabled())
- return;
-
/* slab->obj_exts might not be NULL if it was created for MEMCG accounting. */
if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
return;
@@ -2119,6 +2121,14 @@ alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
}
}

+static inline void
+alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
+ int objects)
+{
+ if (mem_alloc_profiling_enabled())
+ __alloc_tagging_slab_free_hook(s, slab, p, objects);
+}
+
#else /* CONFIG_MEM_ALLOC_PROFILING */

static inline void
--
2.48.1.262.g85cc9f2d1e-goog