Re: [PATCH v3 21/21] mm/slub: cleanup and repurpose some stat items

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Jan 16, 2026 at 6:41 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> 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
> flushing sheaves (counted by SHEAF_FLUSH) no longer also increments
> FREE_SLOWPATH.

nit: I think I understand what you mean but "no longer also
increments" sounds wrong. Maybe repharase as "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: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 77 +++++++++++++++++----------------------------------------------
>  1 file changed, 21 insertions(+), 56 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index c12e90cb2fca..d73ad44fa046 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -330,33 +330,19 @@ 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 */
> @@ -4347,8 +4333,10 @@ 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;
> @@ -4371,6 +4359,7 @@ 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;
>                 }
>         }
> @@ -4379,7 +4368,7 @@ 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;
>  }
> @@ -4451,7 +4440,7 @@ 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;
>
> @@ -5111,8 +5100,6 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>         unsigned long flags;
>         bool on_node_partial;
>
> -       stat(s, FREE_SLOWPATH);

After moving the above accounting to the callers I think there are
several callers which won't account it anymore:
- free_deferred_objects
- memcg_alloc_abort_single
- slab_free_after_rcu_debug
- ___cache_free

Am I missing something or is that intentional?

> -
>         if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
>                 free_to_partial_list(s, slab, head, tail, cnt, addr);
>                 return;
> @@ -5416,7 +5403,7 @@ 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;
>  }
> @@ -5664,7 +5651,7 @@ 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;
> @@ -5686,10 +5673,12 @@ 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;
> @@ -5784,6 +5773,7 @@ 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
> @@ -5806,8 +5796,10 @@ 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
> @@ -6705,6 +6697,7 @@ 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;
> @@ -8704,33 +8697,19 @@ 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);
> @@ -8806,33 +8785,19 @@ 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
>