Re: [PATCH] slub: avoid list_lock contention from __refill_objects_any()

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

On Thu, Jan 29, 2026 at 10:07:57AM +0100, Vlastimil Babka wrote:
> Kernel test robot has reported a regression in the patch "slab: refill
> sheaves from all nodes". When taken in isolation like this, there is
> indeed a tradeoff - we prefer to use remote objects prior to allocating
> new local slabs. It is replicating a behavior that existed before
> sheaves for replenishing cpu (partial) slabs - now called
> get_from_any_partial() to allocate a single object.
> 
> So the possibility of allocating remote objects is intended even if
> remote accesses are then slower. But the profiles in the report also
> suggested a contention on the list_lock spinlock. And that's something
> we can try to avoid without much tradeoff - if someone else has the
> spin_lock, it's more likely they are allocating from the node than
> freeing to it, so we can skip it even if it means allocating a new local
> slab - contributing to that lock's contention isn't worth it. It should
> not result in partial slabs accumulating on the remote node.
> 
> Thus add an allow_spin parameter to __refill_objects_node() and
> get_partial_node_bulk() to make the attempts from __refill_objects_any()
> use only a trylock.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Link: https://lore.kernel.org/oe-lkp/202601132136.77efd6d7-lkp@intel.com
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

In my testing, this patch improved performance by:

will-it-scale.64.processes +14.2%
will-it-scale.128.processes +9.6%
will-it-scale.192.processes +10.8%
will-it-scale.per_process_ops +11.6%


Tested-by: Hao Li <hao.li@linux.dev>

-- 
Thanks
Hao

> ---
> To be applied on top of:
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-7.0/sheaves
> ---
>  mm/slub.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index eb1f52a79999..ca3db3ae1afb 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3378,7 +3378,8 @@ static inline bool pfmemalloc_match(struct slab *slab, gfp_t gfpflags);
>  
>  static bool get_partial_node_bulk(struct kmem_cache *s,
>  				  struct kmem_cache_node *n,
> -				  struct partial_bulk_context *pc)
> +				  struct partial_bulk_context *pc,
> +				  bool allow_spin)
>  {
>  	struct slab *slab, *slab2;
>  	unsigned int total_free = 0;
> @@ -3390,7 +3391,10 @@ static bool get_partial_node_bulk(struct kmem_cache *s,
>  
>  	INIT_LIST_HEAD(&pc->slabs);
>  
> -	spin_lock_irqsave(&n->list_lock, flags);
> +	if (allow_spin)
> +		spin_lock_irqsave(&n->list_lock, flags);
> +	else if (!spin_trylock_irqsave(&n->list_lock, flags))
> +		return false;
>  
>  	list_for_each_entry_safe(slab, slab2, &n->partial, slab_list) {
>  		struct freelist_counters flc;
> @@ -6544,7 +6548,8 @@ EXPORT_SYMBOL(kmem_cache_free_bulk);
>  
>  static unsigned int
>  __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
> -		      unsigned int max, struct kmem_cache_node *n)
> +		      unsigned int max, struct kmem_cache_node *n,
> +		      bool allow_spin)
>  {
>  	struct partial_bulk_context pc;
>  	struct slab *slab, *slab2;
> @@ -6556,7 +6561,7 @@ __refill_objects_node(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int mi
>  	pc.min_objects = min;
>  	pc.max_objects = max;
>  
> -	if (!get_partial_node_bulk(s, n, &pc))
> +	if (!get_partial_node_bulk(s, n, &pc, allow_spin))
>  		return 0;
>  
>  	list_for_each_entry_safe(slab, slab2, &pc.slabs, slab_list) {
> @@ -6650,7 +6655,8 @@ __refill_objects_any(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min
>  					n->nr_partial <= s->min_partial)
>  				continue;
>  
> -			r = __refill_objects_node(s, p, gfp, min, max, n);
> +			r = __refill_objects_node(s, p, gfp, min, max, n,
> +						  /* allow_spin = */ false);
>  			refilled += r;
>  
>  			if (r >= min) {
> @@ -6691,7 +6697,8 @@ refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
>  		return 0;
>  
>  	refilled = __refill_objects_node(s, p, gfp, min, max,
> -					 get_node(s, local_node));
> +					 get_node(s, local_node),
> +					 /* allow_spin = */ true);
>  	if (refilled >= min)
>  		return refilled;
>  
> 
> ---
> base-commit: 6f1912181ddfcf851a6670b4fa9c7dfdaf3ed46d
> change-id: 20260129-b4-refill_any_trylock-160a31224193
> 
> Best regards,
> -- 
> Vlastimil Babka <vbabka@suse.cz>
>