Re: [PATCH RFC v3 4/8] slab: sheaf prefilling for guaranteed allocations

[Suren Baghdasaryan]

On Mon, Mar 17, 2025 at 7:33 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> Add functions for efficient guaranteed allocations e.g. in a critical
> section that cannot sleep, when the exact number of allocations is not
> known beforehand, but an upper limit can be calculated.
>
> kmem_cache_prefill_sheaf() returns a sheaf containing at least given
> number of objects.
>
> kmem_cache_alloc_from_sheaf() will allocate an object from the sheaf
> and is guaranteed not to fail until depleted.
>
> kmem_cache_return_sheaf() is for giving the sheaf back to the slab
> allocator after the critical section. This will also attempt to refill
> it to cache's sheaf capacity for better efficiency of sheaves handling,
> but it's not stricly necessary to succeed.
>
> kmem_cache_refill_sheaf() can be used to refill a previously obtained
> sheaf to requested size. If the current size is sufficient, it does
> nothing. If the requested size exceeds cache's sheaf_capacity and the
> sheaf's current capacity, the sheaf will be replaced with a new one,
> hence the indirect pointer parameter.
>
> kmem_cache_sheaf_size() can be used to query the current size.
>
> The implementation supports requesting sizes that exceed cache's
> sheaf_capacity, but it is not efficient - such sheaves are allocated
> fresh in kmem_cache_prefill_sheaf() and flushed and freed immediately by
> kmem_cache_return_sheaf(). kmem_cache_refill_sheaf() might be especially
> ineffective when replacing a sheaf with a new one of a larger capacity.
> It is therefore better to size cache's sheaf_capacity accordingly.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  include/linux/slab.h |  16 ++++
>  mm/slub.c            | 228 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 244 insertions(+)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 0e1b25228c77140d05b5b4433c9d7923de36ec05..dd01b67982e856b1b02f4f0e6fc557726e7f02a8 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -829,6 +829,22 @@ void *kmem_cache_alloc_node_noprof(struct kmem_cache *s, gfp_t flags,
>                                    int node) __assume_slab_alignment __malloc;
>  #define kmem_cache_alloc_node(...)     alloc_hooks(kmem_cache_alloc_node_noprof(__VA_ARGS__))
>
> +struct slab_sheaf *
> +kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size);
> +
> +int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
> +               struct slab_sheaf **sheafp, unsigned int size);
> +
> +void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
> +                                      struct slab_sheaf *sheaf);
> +
> +void *kmem_cache_alloc_from_sheaf_noprof(struct kmem_cache *cachep, gfp_t gfp,
> +                       struct slab_sheaf *sheaf) __assume_slab_alignment __malloc;
> +#define kmem_cache_alloc_from_sheaf(...)       \
> +                       alloc_hooks(kmem_cache_alloc_from_sheaf_noprof(__VA_ARGS__))
> +
> +unsigned int kmem_cache_sheaf_size(struct slab_sheaf *sheaf);
> +
>  /*
>   * These macros allow declaring a kmem_buckets * parameter alongside size, which
>   * can be compiled out with CONFIG_SLAB_BUCKETS=n so that a large number of call
> diff --git a/mm/slub.c b/mm/slub.c
> index 83f4395267dccfbc144920baa7d0a85a27fbb1b4..ab3532d5f41045d8268b12ad774541dcd066c4c4 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -443,6 +443,8 @@ struct slab_sheaf {
>         union {
>                 struct rcu_head rcu_head;
>                 struct list_head barn_list;
> +               /* only used for prefilled sheafs */
> +               unsigned int capacity;
>         };
>         struct kmem_cache *cache;
>         unsigned int size;
> @@ -2748,6 +2750,30 @@ static int barn_put_full_sheaf(struct node_barn *barn, struct slab_sheaf *sheaf,
>         return ret;
>  }
>
> +static struct slab_sheaf *barn_get_full_or_empty_sheaf(struct node_barn *barn)
> +{
> +       struct slab_sheaf *sheaf = NULL;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&barn->lock, flags);
> +
> +       if (barn->nr_full) {
> +               sheaf = list_first_entry(&barn->sheaves_full, struct slab_sheaf,
> +                                       barn_list);
> +               list_del(&sheaf->barn_list);
> +               barn->nr_full--;
> +       } else if (barn->nr_empty) {
> +               sheaf = list_first_entry(&barn->sheaves_empty,
> +                                        struct slab_sheaf, barn_list);
> +               list_del(&sheaf->barn_list);
> +               barn->nr_empty--;
> +       }
> +
> +       spin_unlock_irqrestore(&barn->lock, flags);
> +
> +       return sheaf;
> +}
> +
>  /*
>   * If a full sheaf is available, return it and put the supplied empty one to
>   * barn. We ignore the limit on empty sheaves as the number of sheaves doesn't
> @@ -4844,6 +4870,208 @@ void *kmem_cache_alloc_node_noprof(struct kmem_cache *s, gfp_t gfpflags, int nod
>  }
>  EXPORT_SYMBOL(kmem_cache_alloc_node_noprof);
>
> +/*
> + * returns a sheaf that has least the requested size
> + * when prefilling is needed, do so with given gfp flags
> + *
> + * return NULL if sheaf allocation or prefilling failed
> + */
> +struct slab_sheaf *
> +kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
> +{
> +       struct slub_percpu_sheaves *pcs;
> +       struct slab_sheaf *sheaf = NULL;
> +
> +       if (unlikely(size > s->sheaf_capacity)) {
> +               sheaf = kzalloc(struct_size(sheaf, objects, size), gfp);
> +               if (!sheaf)
> +                       return NULL;
> +
> +               sheaf->cache = s;
> +               sheaf->capacity = size;
> +
> +               if (!__kmem_cache_alloc_bulk(s, gfp, size,
> +                                            &sheaf->objects[0])) {
> +                       kfree(sheaf);
> +                       return NULL;
> +               }
> +
> +               sheaf->size = size;
> +
> +               return sheaf;
> +       }
> +
> +       localtry_lock(&s->cpu_sheaves->lock);
> +       pcs = this_cpu_ptr(s->cpu_sheaves);
> +
> +       if (pcs->spare) {
> +               sheaf = pcs->spare;
> +               pcs->spare = NULL;
> +       }
> +
> +       if (!sheaf)
> +               sheaf = barn_get_full_or_empty_sheaf(pcs->barn);
> +
> +       localtry_unlock(&s->cpu_sheaves->lock);
> +
> +       if (!sheaf)
> +               sheaf = alloc_empty_sheaf(s, gfp);
> +
> +       if (sheaf && sheaf->size < size) {
> +               if (refill_sheaf(s, sheaf, gfp)) {
> +                       sheaf_flush_unused(s, sheaf);
> +                       free_empty_sheaf(s, sheaf);
> +                       sheaf = NULL;
> +               }
> +       }
> +
> +       if (sheaf)
> +               sheaf->capacity = s->sheaf_capacity;
> +
> +       return sheaf;
> +}
> +
> +/*
> + * Use this to return a sheaf obtained by kmem_cache_prefill_sheaf()
> + *
> + * If the sheaf cannot simply become the percpu spare sheaf, but there's space
> + * for a full sheaf in the barn, we try to refill the sheaf back to the cache's
> + * sheaf_capacity to avoid handling partially full sheaves.
> + *
> + * If the refill fails because gfp is e.g. GFP_NOWAIT, or the barn is full, the
> + * sheaf is instead flushed and freed.
> + */
> +void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
> +                            struct slab_sheaf *sheaf)
> +{
> +       struct slub_percpu_sheaves *pcs;
> +       bool refill = false;
> +       struct node_barn *barn;
> +
> +       if (unlikely(sheaf->capacity != s->sheaf_capacity)) {
> +               sheaf_flush_unused(s, sheaf);
> +               kfree(sheaf);
> +               return;
> +       }
> +
> +       localtry_lock(&s->cpu_sheaves->lock);
> +       pcs = this_cpu_ptr(s->cpu_sheaves);
> +
> +       if (!pcs->spare) {
> +               pcs->spare = sheaf;
> +               sheaf = NULL;
> +       } else if (data_race(pcs->barn->nr_full) < MAX_FULL_SHEAVES) {
> +               barn = pcs->barn;
> +               refill = true;
> +       }
> +
> +       localtry_unlock(&s->cpu_sheaves->lock);
> +
> +       if (!sheaf)
> +               return;
> +
> +       /*
> +        * if the barn is full of full sheaves or we fail to refill the sheaf,
> +        * simply flush and free it
> +        */
> +       if (!refill || refill_sheaf(s, sheaf, gfp)) {
> +               sheaf_flush_unused(s, sheaf);
> +               free_empty_sheaf(s, sheaf);
> +               return;
> +       }
> +
> +       /* we racily determined the sheaf would fit, so now force it */
> +       barn_put_full_sheaf(barn, sheaf, true);
> +}
> +
> +/*
> + * refill a sheaf previously returned by kmem_cache_prefill_sheaf to at least
> + * the given size
> + *
> + * the sheaf might be replaced by a new one when requesting more than
> + * s->sheaf_capacity objects if such replacement is necessary, but the refill
> + * fails (returning -ENOMEM), the existing sheaf is left intact
> + *
> + * In practice we always refill to full sheaf's capacity.
> + */
> +int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
> +                           struct slab_sheaf **sheafp, unsigned int size)

nit: Would returning a refilled sheaf be a slightly better API than
passing pointer to a pointer?

> +{
> +       struct slab_sheaf *sheaf;
> +
> +       /*
> +        * TODO: do we want to support *sheaf == NULL to be equivalent of
> +        * kmem_cache_prefill_sheaf() ?
> +        */
> +       if (!sheafp || !(*sheafp))
> +               return -EINVAL;
> +
> +       sheaf = *sheafp;
> +       if (sheaf->size >= size)
> +               return 0;
> +
> +       if (likely(sheaf->capacity >= size)) {
> +               if (likely(sheaf->capacity == s->sheaf_capacity))
> +                       return refill_sheaf(s, sheaf, gfp);
> +
> +               if (!__kmem_cache_alloc_bulk(s, gfp, sheaf->capacity - sheaf->size,
> +                                            &sheaf->objects[sheaf->size])) {
> +                       return -ENOMEM;
> +               }
> +               sheaf->size = sheaf->capacity;
> +
> +               return 0;
> +       }
> +
> +       /*
> +        * We had a regular sized sheaf and need an oversize one, or we had an
> +        * oversize one already but need a larger one now.
> +        * This should be a very rare path so let's not complicate it.
> +        */
> +       sheaf = kmem_cache_prefill_sheaf(s, gfp, size);
> +       if (!sheaf)
> +               return -ENOMEM;
> +
> +       kmem_cache_return_sheaf(s, gfp, *sheafp);
> +       *sheafp = sheaf;
> +       return 0;
> +}
> +
> +/*
> + * Allocate from a sheaf obtained by kmem_cache_prefill_sheaf()
> + *
> + * Guaranteed not to fail as many allocations as was the requested size.
> + * After the sheaf is emptied, it fails - no fallback to the slab cache itself.
> + *
> + * The gfp parameter is meant only to specify __GFP_ZERO or __GFP_ACCOUNT
> + * memcg charging is forced over limit if necessary, to avoid failure.
> + */
> +void *
> +kmem_cache_alloc_from_sheaf_noprof(struct kmem_cache *s, gfp_t gfp,
> +                                  struct slab_sheaf *sheaf)
> +{
> +       void *ret = NULL;
> +       bool init;
> +
> +       if (sheaf->size == 0)
> +               goto out;
> +
> +       ret = sheaf->objects[--sheaf->size];
> +
> +       init = slab_want_init_on_alloc(gfp, s);
> +
> +       /* add __GFP_NOFAIL to force successful memcg charging */
> +       slab_post_alloc_hook(s, NULL, gfp | __GFP_NOFAIL, 1, &ret, init, s->object_size);
> +out:
> +       trace_kmem_cache_alloc(_RET_IP_, ret, s, gfp, NUMA_NO_NODE);
> +
> +       return ret;
> +}
> +
> +unsigned int kmem_cache_sheaf_size(struct slab_sheaf *sheaf)
> +{
> +       return sheaf->size;
> +}
>  /*
>   * To avoid unnecessary overhead, we pass through large allocation requests
>   * directly to the page allocator. We use __GFP_COMP, because we will need to
>
> --
> 2.48.1
>