Re: [PATCH 2/3] slab: create barns for online memoryless nodes

[Hao Li <hao.li@linux.dev>]

On Wed, Mar 11, 2026 at 09:25:56AM +0100, Vlastimil Babka (SUSE) wrote:
> Ming Lei has reported [1] a performance regression due to replacing cpu
> (partial) slabs with sheaves. With slub stats enabled, a large amount of
> slowpath allocations were observed. The affected system has 8 online
> NUMA nodes but only 2 have memory.
> 
> For sheaves to work effectively on given cpu, its NUMA node has to have
> struct node_barn allocated. Those are currently only allocated on nodes
> with memory (N_MEMORY) where kmem_cache_node also exist as the goal is
> to cache only node-local objects. But in order to have good performance
> on a memoryless node, we need its barn to exist and use sheaves to cache
> non-local objects (as no local objects can exist anyway).
> 
> Therefore change the implementation to allocate barns on all online
> nodes, tracked in a new nodemask slab_barn_nodes. Also add a cpu hotplug
> callback as that's when a memoryless node can become online.
> 
> Change rcu_sheaf->node assignment to numa_node_id() so it's returned to
> the barn of the local cpu's (potentially memoryless) node, and not to
> the nearest node with memory anymore.
> 
> Reported-by: Ming Lei <ming.lei@redhat.com>
> Link: https://lore.kernel.org/all/aZ0SbIqaIkwoW2mB@fedora/ [1]
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---
>  mm/slub.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 59 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 609a183f8533..d8496b37e364 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
[...]
>  
>  	/*
> @@ -7597,7 +7648,7 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
>  	if (slab_state == DOWN || !cache_has_sheaves(s))
>  		return 1;
>  
> -	for_each_node_mask(node, slab_nodes) {
> +	for_each_node_mask(node, slab_barn_nodes) {
>  		struct node_barn *barn;
>  
>  		barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
> @@ -8250,6 +8301,7 @@ static int slab_mem_going_online_callback(int nid)
>  	 * and barn initialized for the new node.
>  	 */
>  	node_set(nid, slab_nodes);
> +	node_set(nid, slab_barn_nodes);

I had a somewhat related question here.

During memory hotplug, we call node_set() on slab_nodes when memory is brought
online, but we do not seem to call node_clear() when memory is taken offline. I
was wondering what the reasoning behind this is.

That also made me wonder about a related case. If I am understanding this
correctly, even if all memory of a node has been offlined, slab_nodes would
still make it appear that the node has memory, even though in reality it no
longer does. If so, then in patch 3, the condition
"if (unlikely(!node_isset(numa_node, slab_nodes)))" in can_free_to_pcs() seems
would cause the object free path to skip sheaves.

-- 
Thanks,
Hao