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

[Vlastimil Babka <vbabka@suse.cz>]

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>
---
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>