Re: [PATCH 0/3] slab: support memoryless nodes with sheaves

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

On 3/11/26 09:25, Vlastimil Babka (SUSE) wrote:
> This is the draft patch from [1] turned into a proper series with
> incremental changes. It's based on v7.0-rc3. It's too intrusive for a
> 7.0 hotfix, so we'll only be able to fix/reduce the regression in 7.1. I
> hope it's acceptable given it's a non-standard configuration, 7.0 is not
> a LTS, and it's a perf regression, not functionality.
> 
> Ming can you please retest this on top of v7.0-rc3, which already has
> fb1091febd66 ("mm/slab: allow sheaf refill if blocking is not
> allowed"). Separate data point for v7.0-rc3 could be also useful.
> 
> [1] https://lore.kernel.org/all/c6a01f7e-c6eb-454b-9b9e-734526dd659d@kernel.org/
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---
> Vlastimil Babka (SUSE) (3):
>       slab: decouple pointer to barn from kmem_cache_node
>       slab: create barns for online memoryless nodes
>       slab: free remote objects to sheaves on memoryless nodes
> 
>  mm/slab.h |   7 +-
>  mm/slub.c | 256 +++++++++++++++++++++++++++++++++++++++++++++-----------------
>  2 files changed, 191 insertions(+), 72 deletions(-)
> ---
> base-commit: 1f318b96cc84d7c2ab792fcc0bfd42a7ca890681
> change-id: 20260311-b4-slab-memoryless-barns-fad64172ba05
> 
> Best regards,

Range-diff in slab/for-7.1/sheaves after applying Harry's feedback:

  2:  cc67056e94f1 ! 472:  b002755da434 slab: decouple pointer to barn from kmem_cache_node
    @@ Commit message
     
         Link: https://patch.msgid.link/20260311-b4-slab-memoryless-barns-v1-1-70ab850be4ce@kernel.org
         Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
    +    Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
     
      ## mm/slab.h ##
     @@ mm/slab.h: struct kmem_cache_order_objects {
    @@ mm/slub.c: struct kmem_cache_node {
      /*
     - * Get the barn of the current cpu's closest memory node. It may not exist on
     - * systems with memoryless nodes but without CONFIG_HAVE_MEMORYLESS_NODES
    -+ * Get the barn of the current cpu's memory node. It may be a memoryless node.
    ++ * Get the barn of the current cpu's NUMA node. It may be a memoryless node.
       */
      static inline struct node_barn *get_barn(struct kmem_cache *s)
      {
    @@ mm/slub.c: struct kmem_cache_node {
     -          return NULL;
     -
     -  return n->barn;
    -+  return get_barn_node(s, numa_node_id());
    ++  return get_barn_node(s, numa_mem_id());
      }
      
      /*
  3:  285bca63cf15 ! 473:  f811cc3d9f6e slab: create barns for online memoryless nodes
    @@ Commit message
         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.
    +    Change both get_barn() and 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.
    +
    +    On systems with CONFIG_HAVE_MEMORYLESS_NODES=y (which are not the main
    +    target of this change) barns did not exist on memoryless nodes, but
    +    get_barn() using numa_mem_id() meant a barn was returned from the
    +    nearest node with memory. This works, but the barn lock contention
    +    increases with every such memoryless node. With this change, barn will
    +    be allocated also on the memoryless node, reducing this contention in
    +    exchange for increased memory consumption.
     
         Reported-by: Ming Lei <ming.lei@redhat.com>
         Link: https://lore.kernel.org/all/aZ0SbIqaIkwoW2mB@fedora/ [1]
         Link: https://patch.msgid.link/20260311-b4-slab-memoryless-barns-v1-2-70ab850be4ce@kernel.org
         Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
    +    Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
     
      ## mm/slub.c ##
    +@@ mm/slub.c: static inline struct node_barn *get_barn_node(struct kmem_cache *s, int node)
    +  */
    + static inline struct node_barn *get_barn(struct kmem_cache *s)
    + {
    +-  return get_barn_node(s, numa_mem_id());
    ++  return get_barn_node(s, numa_node_id());
    + }
    + 
    + /*
     @@ mm/slub.c: static inline struct node_barn *get_barn(struct kmem_cache *s)
       */
      static nodemask_t slab_nodes;
  4:  1fe49af3aa46 ! 474:  86e18f36844f slab: free remote objects to sheaves on memoryless nodes
    @@ Commit message
     
         Link: https://patch.msgid.link/20260311-b4-slab-memoryless-barns-v1-3-70ab850be4ce@kernel.org
         Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
    +    Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
     
      ## mm/slub.c ##
     @@ mm/slub.c: bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)