[PATCH v2 3/3] mm/slub: Use percpu partial free counter

[Xunlei Pang]

The only concern of introducing partial counter is that,
partial_free_objs may cause atomic operation contention
in case of same SLUB concurrent __slab_free().

This patch changes it to be a percpu counter to avoid that.

Co-developed-by: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>
---
 mm/slab.h |  2 +-
 mm/slub.c | 38 +++++++++++++++++++++++++++++++-------
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index c85e2fa..a709a70 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -616,7 +616,7 @@ struct kmem_cache_node {
 #ifdef CONFIG_SLUB
 	unsigned long nr_partial;
 	struct list_head partial;
-	atomic_long_t partial_free_objs;
+	atomic_long_t __percpu *partial_free_objs;
 	atomic_long_t partial_total_objs;
 #ifdef CONFIG_SLUB_DEBUG
 	atomic_long_t nr_slabs;
diff --git a/mm/slub.c b/mm/slub.c
index 25a4421..f6fc60b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1775,11 +1775,21 @@ static void discard_slab(struct kmem_cache *s, struct page *page)
 /*
  * Management of partially allocated slabs.
  */
+static inline long get_partial_free(struct kmem_cache_node *n)
+{
+	long nr = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		nr += atomic_long_read(per_cpu_ptr(n->partial_free_objs, cpu));
+
+	return nr;
+}
 
 static inline void
 __update_partial_free(struct kmem_cache_node *n, long delta)
 {
-	atomic_long_add(delta, &n->partial_free_objs);
+	atomic_long_add(delta, this_cpu_ptr(n->partial_free_objs));
 }
 
 static inline void
@@ -2429,12 +2439,12 @@ static unsigned long partial_counter(struct kmem_cache_node *n,
 	unsigned long ret = 0;
 
 	if (item == PARTIAL_FREE) {
-		ret = atomic_long_read(&n->partial_free_objs);
+		ret = get_partial_free(n);
 	} else if (item == PARTIAL_TOTAL) {
 		ret = atomic_long_read(&n->partial_total_objs);
 	} else if (item == PARTIAL_INUSE) {
 		ret = atomic_long_read(&n->partial_total_objs) -
-		atomic_long_read(&n->partial_free_objs);
+				get_partial_free(n);
 		if ((long)ret < 0)
 			ret = 0;
 	}
@@ -3390,19 +3400,28 @@ static inline int calculate_order(unsigned int size)
 	return -ENOSYS;
 }
 
-static void
+static int
 init_kmem_cache_node(struct kmem_cache_node *n)
 {
+	int cpu;
+
 	n->nr_partial = 0;
 	spin_lock_init(&n->list_lock);
 	INIT_LIST_HEAD(&n->partial);
-	atomic_long_set(&n->partial_free_objs, 0);
+
+	n->partial_free_objs = alloc_percpu(atomic_long_t);
+	if (!n->partial_free_objs)
+		return -ENOMEM;
+	for_each_possible_cpu(cpu)
+		atomic_long_set(per_cpu_ptr(n->partial_free_objs, cpu), 0);
 	atomic_long_set(&n->partial_total_objs, 0);
 #ifdef CONFIG_SLUB_DEBUG
 	atomic_long_set(&n->nr_slabs, 0);
 	atomic_long_set(&n->total_objects, 0);
 	INIT_LIST_HEAD(&n->full);
 #endif
+
+	return 0;
 }
 
 static inline int alloc_kmem_cache_cpus(struct kmem_cache *s)
@@ -3463,7 +3482,7 @@ static void early_kmem_cache_node_alloc(int node)
 	page->inuse = 1;
 	page->frozen = 0;
 	kmem_cache_node->node[node] = n;
-	init_kmem_cache_node(n);
+	BUG_ON(init_kmem_cache_node(n) < 0);
 	inc_slabs_node(kmem_cache_node, node, page->objects);
 
 	/*
@@ -3481,6 +3500,7 @@ static void free_kmem_cache_nodes(struct kmem_cache *s)
 
 	for_each_kmem_cache_node(s, node, n) {
 		s->node[node] = NULL;
+		free_percpu(n->partial_free_objs);
 		kmem_cache_free(kmem_cache_node, n);
 	}
 }
@@ -3511,7 +3531,11 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
 			return 0;
 		}
 
-		init_kmem_cache_node(n);
+		if (init_kmem_cache_node(n) < 0) {
+			free_kmem_cache_nodes(s);
+			return 0;
+		}
+
 		s->node[node] = n;
 	}
 	return 1;
-- 
1.8.3.1