[PATCH v4 22/22] mm/slub: cleanup and repurpose some stat items

Vlastimil Babka
2026-01-23T06:54:18Z

A number of stat items related to cpu slabs became unused, remove them.

Two of those were ALLOC_FASTPATH and FREE_FASTPATH. But instead of
removing those, use them instead of ALLOC_PCS and FREE_PCS, since
sheaves are the new (and only) fastpaths, Remove the recently added
_PCS variants instead.

Change where FREE_SLOWPATH is counted so that it only counts freeing of
objects by slab users that (for whatever reason) do not go to a percpu
sheaf, and not all (including internal) callers of __slab_free(). Thus
sheaf flushing (already counted by SHEAF_FLUSH) does not affect
FREE_SLOWPATH anymore. This matches how ALLOC_SLOWPATH doesn't count
sheaf refills (counted by SHEAF_REFILL).

Reviewed-by: Hao Li <hao.li@linux.dev>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/slub.c | 83 ++++++++++++++++++++-------------------------------------------
1 file changed, 26 insertions(+), 57 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 369fb9bbdb75..b07a27a3ca28 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -335,33 +335,19 @@ enum add_mode {
};

enum stat_item {
- ALLOC_PCS, /* Allocation from percpu sheaf */
- ALLOC_FASTPATH, /* Allocation from cpu slab */
- ALLOC_SLOWPATH, /* Allocation by getting a new cpu slab */
- FREE_PCS, /* Free to percpu sheaf */
+ ALLOC_FASTPATH, /* Allocation from percpu sheaves */
+ ALLOC_SLOWPATH, /* Allocation from partial or new slab */
FREE_RCU_SHEAF, /* Free to rcu_free sheaf */
FREE_RCU_SHEAF_FAIL, /* Failed to free to a rcu_free sheaf */
- FREE_FASTPATH, /* Free to cpu slab */
- FREE_SLOWPATH, /* Freeing not to cpu slab */
+ FREE_FASTPATH, /* Free to percpu sheaves */
+ FREE_SLOWPATH, /* Free to a slab */
FREE_ADD_PARTIAL, /* Freeing moves slab to partial list */
FREE_REMOVE_PARTIAL, /* Freeing removes last object */
- ALLOC_FROM_PARTIAL, /* Cpu slab acquired from node partial list */
- ALLOC_SLAB, /* Cpu slab acquired from page allocator */
- ALLOC_REFILL, /* Refill cpu slab from slab freelist */
- ALLOC_NODE_MISMATCH, /* Switching cpu slab */
+ ALLOC_SLAB, /* New slab acquired from page allocator */
+ ALLOC_NODE_MISMATCH, /* Requested node different from cpu sheaf */
FREE_SLAB, /* Slab freed to the page allocator */
- CPUSLAB_FLUSH, /* Abandoning of the cpu slab */
- DEACTIVATE_FULL, /* Cpu slab was full when deactivated */
- DEACTIVATE_EMPTY, /* Cpu slab was empty when deactivated */
- DEACTIVATE_REMOTE_FREES,/* Slab contained remotely freed objects */
- DEACTIVATE_BYPASS, /* Implicit deactivation */
ORDER_FALLBACK, /* Number of times fallback was necessary */
- CMPXCHG_DOUBLE_CPU_FAIL,/* Failures of this_cpu_cmpxchg_double */
CMPXCHG_DOUBLE_FAIL, /* Failures of slab freelist update */
- CPU_PARTIAL_ALLOC, /* Used cpu partial on alloc */
- CPU_PARTIAL_FREE, /* Refill cpu partial on free */
- CPU_PARTIAL_NODE, /* Refill cpu partial from node partial */
- CPU_PARTIAL_DRAIN, /* Drain cpu partial to node partial */
SHEAF_FLUSH, /* Objects flushed from a sheaf */
SHEAF_REFILL, /* Objects refilled to a sheaf */
SHEAF_ALLOC, /* Allocation of an empty sheaf */
@@ -4350,8 +4336,10 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp, int node)
* We assume the percpu sheaves contain only local objects although it's
* not completely guaranteed, so we verify later.
*/
- if (unlikely(node_requested && node != numa_mem_id()))
+ if (unlikely(node_requested && node != numa_mem_id())) {
+ stat(s, ALLOC_NODE_MISMATCH);
return NULL;
+ }

if (!local_trylock(&s->cpu_sheaves->lock))
return NULL;
@@ -4374,6 +4362,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp, int node)
*/
if (page_to_nid(virt_to_page(object)) != node) {
local_unlock(&s->cpu_sheaves->lock);
+ stat(s, ALLOC_NODE_MISMATCH);
return NULL;
}
}
@@ -4382,7 +4371,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp, int node)

local_unlock(&s->cpu_sheaves->lock);

- stat(s, ALLOC_PCS);
+ stat(s, ALLOC_FASTPATH);

return object;
}
@@ -4454,7 +4443,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, gfp_t gfp, size_t size,

local_unlock(&s->cpu_sheaves->lock);

- stat_add(s, ALLOC_PCS, batch);
+ stat_add(s, ALLOC_FASTPATH, batch);

allocated += batch;

@@ -5117,8 +5106,6 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
unsigned long flags;
bool on_node_partial;

- stat(s, FREE_SLOWPATH);
-
if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
free_to_partial_list(s, slab, head, tail, cnt, addr);
return;
@@ -5422,7 +5409,7 @@ bool free_to_pcs(struct kmem_cache *s, void *object, bool allow_spin)

local_unlock(&s->cpu_sheaves->lock);

- stat(s, FREE_PCS);
+ stat(s, FREE_FASTPATH);

return true;
}
@@ -5687,7 +5674,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)

local_unlock(&s->cpu_sheaves->lock);

- stat_add(s, FREE_PCS, batch);
+ stat_add(s, FREE_FASTPATH, batch);

if (batch < size) {
p += batch;
@@ -5709,10 +5696,12 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
*/
fallback:
__kmem_cache_free_bulk(s, size, p);
+ stat_add(s, FREE_SLOWPATH, size);

flush_remote:
if (remote_nr) {
__kmem_cache_free_bulk(s, remote_nr, &remote_objects[0]);
+ stat_add(s, FREE_SLOWPATH, remote_nr);
if (i < size) {
remote_nr = 0;
goto next_remote_batch;
@@ -5766,6 +5755,7 @@ static void free_deferred_objects(struct irq_work *work)
set_freepointer(s, x, NULL);

__slab_free(s, slab, x, x, 1, _THIS_IP_);
+ stat(s, FREE_SLOWPATH);
}
}

@@ -5807,6 +5797,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
}

__slab_free(s, slab, object, object, 1, addr);
+ stat(s, FREE_SLOWPATH);
}

#ifdef CONFIG_MEMCG
@@ -5829,8 +5820,10 @@ void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head,
* With KASAN enabled slab_free_freelist_hook modifies the freelist
* to remove objects, whose reuse must be delayed.
*/
- if (likely(slab_free_freelist_hook(s, &head, &tail, &cnt)))
+ if (likely(slab_free_freelist_hook(s, &head, &tail, &cnt))) {
__slab_free(s, slab, head, tail, cnt, addr);
+ stat_add(s, FREE_SLOWPATH, cnt);
+ }
}

#ifdef CONFIG_SLUB_RCU_DEBUG
@@ -5855,8 +5848,10 @@ static void slab_free_after_rcu_debug(struct rcu_head *rcu_head)
return;

/* resume freeing */
- if (slab_free_hook(s, object, slab_want_init_on_free(s), true))
+ if (slab_free_hook(s, object, slab_want_init_on_free(s), true)) {
__slab_free(s, slab, object, object, 1, _THIS_IP_);
+ stat(s, FREE_SLOWPATH);
+ }
}
#endif /* CONFIG_SLUB_RCU_DEBUG */

@@ -5864,6 +5859,7 @@ static void slab_free_after_rcu_debug(struct rcu_head *rcu_head)
void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
{
__slab_free(cache, virt_to_slab(x), x, x, 1, addr);
+ stat(cache, FREE_SLOWPATH);
}
#endif

@@ -6733,6 +6729,7 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
i = refill_objects(s, p, flags, size, size);
if (i < size)
goto error;
+ stat_add(s, ALLOC_SLOWPATH, i);
}

return i;
@@ -8736,33 +8733,19 @@ static ssize_t text##_store(struct kmem_cache *s, \
} \
SLAB_ATTR(text); \

-STAT_ATTR(ALLOC_PCS, alloc_cpu_sheaf);
STAT_ATTR(ALLOC_FASTPATH, alloc_fastpath);
STAT_ATTR(ALLOC_SLOWPATH, alloc_slowpath);
-STAT_ATTR(FREE_PCS, free_cpu_sheaf);
STAT_ATTR(FREE_RCU_SHEAF, free_rcu_sheaf);
STAT_ATTR(FREE_RCU_SHEAF_FAIL, free_rcu_sheaf_fail);
STAT_ATTR(FREE_FASTPATH, free_fastpath);
STAT_ATTR(FREE_SLOWPATH, free_slowpath);
STAT_ATTR(FREE_ADD_PARTIAL, free_add_partial);
STAT_ATTR(FREE_REMOVE_PARTIAL, free_remove_partial);
-STAT_ATTR(ALLOC_FROM_PARTIAL, alloc_from_partial);
STAT_ATTR(ALLOC_SLAB, alloc_slab);
-STAT_ATTR(ALLOC_REFILL, alloc_refill);
STAT_ATTR(ALLOC_NODE_MISMATCH, alloc_node_mismatch);
STAT_ATTR(FREE_SLAB, free_slab);
-STAT_ATTR(CPUSLAB_FLUSH, cpuslab_flush);
-STAT_ATTR(DEACTIVATE_FULL, deactivate_full);
-STAT_ATTR(DEACTIVATE_EMPTY, deactivate_empty);
-STAT_ATTR(DEACTIVATE_REMOTE_FREES, deactivate_remote_frees);
-STAT_ATTR(DEACTIVATE_BYPASS, deactivate_bypass);
STAT_ATTR(ORDER_FALLBACK, order_fallback);
-STAT_ATTR(CMPXCHG_DOUBLE_CPU_FAIL, cmpxchg_double_cpu_fail);
STAT_ATTR(CMPXCHG_DOUBLE_FAIL, cmpxchg_double_fail);
-STAT_ATTR(CPU_PARTIAL_ALLOC, cpu_partial_alloc);
-STAT_ATTR(CPU_PARTIAL_FREE, cpu_partial_free);
-STAT_ATTR(CPU_PARTIAL_NODE, cpu_partial_node);
-STAT_ATTR(CPU_PARTIAL_DRAIN, cpu_partial_drain);
STAT_ATTR(SHEAF_FLUSH, sheaf_flush);
STAT_ATTR(SHEAF_REFILL, sheaf_refill);
STAT_ATTR(SHEAF_ALLOC, sheaf_alloc);
@@ -8838,33 +8821,19 @@ static struct attribute *slab_attrs[] = {
&remote_node_defrag_ratio_attr.attr,
#endif
#ifdef CONFIG_SLUB_STATS
- &alloc_cpu_sheaf_attr.attr,
&alloc_fastpath_attr.attr,
&alloc_slowpath_attr.attr,
- &free_cpu_sheaf_attr.attr,
&free_rcu_sheaf_attr.attr,
&free_rcu_sheaf_fail_attr.attr,
&free_fastpath_attr.attr,
&free_slowpath_attr.attr,
&free_add_partial_attr.attr,
&free_remove_partial_attr.attr,
- &alloc_from_partial_attr.attr,
&alloc_slab_attr.attr,
- &alloc_refill_attr.attr,
&alloc_node_mismatch_attr.attr,
&free_slab_attr.attr,
- &cpuslab_flush_attr.attr,
- &deactivate_full_attr.attr,
- &deactivate_empty_attr.attr,
- &deactivate_remote_frees_attr.attr,
- &deactivate_bypass_attr.attr,
&order_fallback_attr.attr,
&cmpxchg_double_fail_attr.attr,
- &cmpxchg_double_cpu_fail_attr.attr,
- &cpu_partial_alloc_attr.attr,
- &cpu_partial_free_attr.attr,
- &cpu_partial_node_attr.attr,
- &cpu_partial_drain_attr.attr,
&sheaf_flush_attr.attr,
&sheaf_refill_attr.attr,
&sheaf_alloc_attr.attr,

--
2.52.0