Re: [PATCH RFC v2 04/20] slab: add sheaves to most caches

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Jan 12, 2026 at 3:17 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> In the first step to replace cpu (partial) slabs with sheaves, enable
> sheaves for almost all caches. Treat args->sheaf_capacity as a minimum,
> and calculate sheaf capacity with a formula that roughly follows the
> formula for number of objects in cpu partial slabs in set_cpu_partial().
>
> This should achieve roughly similar contention on the barn spin lock as
> there's currently for node list_lock without sheaves, to make
> benchmarking results comparable. It can be further tuned later.
>
> Don't enable sheaves for bootstrap caches as that wouldn't work. In
> order to recognize them by SLAB_NO_OBJ_EXT, make sure the flag exists
> even for !CONFIG_SLAB_OBJ_EXT.
>
> This limitation will be lifted for kmalloc caches after the necessary
> bootstrapping changes.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

One nit but otherwise LGTM.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  include/linux/slab.h |  6 ------
>  mm/slub.c            | 51 +++++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 47 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 2482992248dc..2682ee57ec90 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -57,9 +57,7 @@ enum _slab_flag_bits {
>  #endif
>         _SLAB_OBJECT_POISON,
>         _SLAB_CMPXCHG_DOUBLE,
> -#ifdef CONFIG_SLAB_OBJ_EXT
>         _SLAB_NO_OBJ_EXT,
> -#endif
>         _SLAB_FLAGS_LAST_BIT
>  };
>
> @@ -238,11 +236,7 @@ enum _slab_flag_bits {
>  #define SLAB_TEMPORARY         SLAB_RECLAIM_ACCOUNT    /* Objects are short-lived */
>
>  /* Slab created using create_boot_cache */
> -#ifdef CONFIG_SLAB_OBJ_EXT
>  #define SLAB_NO_OBJ_EXT                __SLAB_FLAG_BIT(_SLAB_NO_OBJ_EXT)
> -#else
> -#define SLAB_NO_OBJ_EXT                __SLAB_FLAG_UNUSED
> -#endif
>
>  /*
>   * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
> diff --git a/mm/slub.c b/mm/slub.c
> index 8ffeb3ab3228..6e05e3cc5c49 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7857,6 +7857,48 @@ static void set_cpu_partial(struct kmem_cache *s)
>  #endif
>  }
>
> +static unsigned int calculate_sheaf_capacity(struct kmem_cache *s,
> +                                            struct kmem_cache_args *args)
> +
> +{
> +       unsigned int capacity;
> +       size_t size;
> +
> +
> +       if (IS_ENABLED(CONFIG_SLUB_TINY) || s->flags & SLAB_DEBUG_FLAGS)
> +               return 0;
> +
> +       /* bootstrap caches can't have sheaves for now */
> +       if (s->flags & SLAB_NO_OBJ_EXT)
> +               return 0;
> +
> +       /*
> +        * For now we use roughly similar formula (divided by two as there are
> +        * two percpu sheaves) as what was used for percpu partial slabs, which
> +        * should result in similar lock contention (barn or list_lock)
> +        */
> +       if (s->size >= PAGE_SIZE)
> +               capacity = 4;
> +       else if (s->size >= 1024)
> +               capacity = 12;
> +       else if (s->size >= 256)
> +               capacity = 26;
> +       else
> +               capacity = 60;
> +
> +       /* Increment capacity to make sheaf exactly a kmalloc size bucket */
> +       size = struct_size_t(struct slab_sheaf, objects, capacity);
> +       size = kmalloc_size_roundup(size);
> +       capacity = (size - struct_size_t(struct slab_sheaf, objects, 0)) / sizeof(void *);
> +
> +       /*
> +        * Respect an explicit request for capacity that's typically motivated by
> +        * expected maximum size of kmem_cache_prefill_sheaf() to not end up
> +        * using low-performance oversize sheaves
> +        */
> +       return max(capacity, args->sheaf_capacity);
> +}
> +
>  /*
>   * calculate_sizes() determines the order and the distribution of data within
>   * a slab object.
> @@ -7991,6 +8033,10 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>         if (s->flags & SLAB_RECLAIM_ACCOUNT)
>                 s->allocflags |= __GFP_RECLAIMABLE;
>
> +       /* kmalloc caches need extra care to support sheaves */
> +       if (!is_kmalloc_cache(s))

nit: All the checks for the cases when sheaves should not be used
(like SLAB_DEBUG_FLAGS and SLAB_NO_OBJ_EXT) are done inside
calculate_sheaf_capacity(). Only this is_kmalloc_cache() one is here.
It would be nice to have all of them in the same place but maybe you
have a reason for keeping it here?

> +               s->sheaf_capacity = calculate_sheaf_capacity(s, args);
> +
>         /*
>          * Determine the number of objects per slab
>          */
> @@ -8595,15 +8641,12 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
>
>         set_cpu_partial(s);
>
> -       if (args->sheaf_capacity && !IS_ENABLED(CONFIG_SLUB_TINY)
> -                                       && !(s->flags & SLAB_DEBUG_FLAGS)) {
> +       if (s->sheaf_capacity) {
>                 s->cpu_sheaves = alloc_percpu(struct slub_percpu_sheaves);
>                 if (!s->cpu_sheaves) {
>                         err = -ENOMEM;
>                         goto out;
>                 }
> -               // TODO: increase capacity to grow slab_sheaf up to next kmalloc size?
> -               s->sheaf_capacity = args->sheaf_capacity;
>         }
>
>  #ifdef CONFIG_NUMA
>
> --
> 2.52.0
>