Re: [PATCH v3 09/21] slab: add optimized sheaf refill from partial list

Harry Yoo
2026-01-20T02:33:44Z

On Fri, Jan 16, 2026 at 03:40:29PM +0100, Vlastimil Babka wrote:
> At this point we have sheaves enabled for all caches, but their refill
> is done via __kmem_cache_alloc_bulk() which relies on cpu (partial)
> slabs - now a redundant caching layer that we are about to remove.
>
> The refill will thus be done from slabs on the node partial list.
> Introduce new functions that can do that in an optimized way as it's
> easier than modifying the __kmem_cache_alloc_bulk() call chain.
>
> Extend struct partial_context so it can return a list of slabs from the
> partial list with the sum of free objects in them within the requested
> min and max.
>
> Introduce get_partial_node_bulk() that removes the slabs from freelist
> and returns them in the list.
>
> Introduce get_freelist_nofreeze() which grabs the freelist without
> freezing the slab.
>
> Introduce alloc_from_new_slab() which can allocate multiple objects from
> a newly allocated slab where we don't need to synchronize with freeing.
> In some aspects it's similar to alloc_single_from_new_slab() but assumes
> the cache is a non-debug one so it can avoid some actions.
>
> Introduce __refill_objects() that uses the functions above to fill an
> array of objects. It has to handle the possibility that the slabs will
> contain more objects that were requested, due to concurrent freeing of
> objects to those slabs. When no more slabs on partial lists are
> available, it will allocate new slabs. It is intended to be only used
> in context where spinning is allowed, so add a WARN_ON_ONCE check there.
>
> Finally, switch refill_sheaf() to use __refill_objects(). Sheaves are
> only refilled from contexts that allow spinning, or even blocking.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/slub.c | 284 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 264 insertions(+), 20 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 9bea8a65e510..dce80463f92c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -246,6 +246,9 @@ struct partial_context {
> gfp_t flags;
> unsigned int orig_size;
> void *object;
> + unsigned int min_objects;
> + unsigned int max_objects;
> + struct list_head slabs;
> };
>
> static inline bool kmem_cache_debug(struct kmem_cache *s)
> @@ -2663,8 +2666,8 @@ static int refill_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf,
> if (!to_fill)
> return 0;
>
> - filled = __kmem_cache_alloc_bulk(s, gfp, to_fill,
> - &sheaf->objects[sheaf->size]);
> + filled = __refill_objects(s, &sheaf->objects[sheaf->size], gfp,
> + to_fill, to_fill);

nit: perhaps handling min and max separately is unnecessary
if it's always min == max? we could have simply one 'count' or 'size'?

Otherwise LGTM!

--
Cheers,
Harry / Hyeonggon