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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/18/26 10:27, Hao Li wrote:
> 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.

Probably nobody took the task the implement the necessary teardown.

> 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.

Maybe the condition should be looking at N_MEMORY then?

Also ideally we should be using N_NORMAL_MEMORY everywhere for slab_nodes.
Oh we actually did, but give that up in commit 1bf47d4195e45.

Note in practice full memory offline of a node can only be achieved if it
was all ZONE_MOVABLE and thus no slab allocations ever happened on it. But
if it has only movable memory, it's practically memoryless for slab
purposes. Maybe the condition should be looking at N_NORMAL_MEMORY then.
That would cover the case when it became offline and also the case when it's
online but with only movable memory?

I don't know if with CONFIG_HAVE_MEMORYLESS_NODES it's possible that
numa_mem_id() (the closest node with memory) would be ZONE_MOVABLE only.
Maybe let's hope not, and not adjust that part?