Track emergency object in rbtree

In the initial implementation emergency objects were tracked on a
per-cache list.  The assumption was that under normal operation we
would never allocate more than a handful of these objects.  So the
cost of walking the list during free was expected to be negligible.

However real world usage has shown that emergency objects tend to
be allocated in batches.  A deadlock will be detected and several
thousand emergency objects will be allocated before the original
blocked slab allocation can complete.

Therefore the original list has been replaced by a red black tree
which is sorted by the memory address of each allocated object.
This bounds the worst case insertion and removal time to O(log n)
which minimize contention on the assoicated spin lock.

Signed-off-by: Brian Behlendorf <[email protected]>

1 files changed, 72 insertions, 26 deletions

diff --git a/module/spl/spl-kmem.c b/module/spl/spl-kmem.c
index 045075cc0..7e68522ad 100644
--- a/module/spl/spl-kmem.c
+++ b/module/spl/spl-kmem.c
@@ -1116,8 +1116,54 @@ spl_slab_reclaim(spl_kmem_cache_t *skc, int count, int flag)
 	SEXIT;
 }
 
+static spl_kmem_emergency_t *
+spl_emergency_search(struct rb_root *root, void *obj)
+{
+	struct rb_node *node = root->rb_node;
+	spl_kmem_emergency_t *ske;
+	unsigned long address = (unsigned long)obj;
+
+	while (node) {
+		ske = container_of(node, spl_kmem_emergency_t, ske_node);
+
+		if (address < (unsigned long)ske->ske_obj)
+			node = node->rb_left;
+		else if (address > (unsigned long)ske->ske_obj)
+			node = node->rb_right;
+		else
+			return ske;
+	}
+
+	return NULL;
+}
+
+static int
+spl_emergency_insert(struct rb_root *root, spl_kmem_emergency_t *ske)
+{
+	struct rb_node **new = &(root->rb_node), *parent = NULL;
+	spl_kmem_emergency_t *ske_tmp;
+	unsigned long address = (unsigned long)ske->ske_obj;
+
+	while (*new) {
+		ske_tmp = container_of(*new, spl_kmem_emergency_t, ske_node);
+
+		parent = *new;
+		if (address < (unsigned long)ske_tmp->ske_obj)
+			new = &((*new)->rb_left);
+		else if (address > (unsigned long)ske_tmp->ske_obj)
+			new = &((*new)->rb_right);
+		else
+			return 0;
+	}
+
+	rb_link_node(&ske->ske_node, parent, new);
+	rb_insert_color(&ske->ske_node, root);
+
+	return 1;
+}
+
 /*
- * Allocate a single emergency object for use by the caller.
+ * Allocate a single emergency object and track it in a red black tree.
  */
 static int
 spl_emergency_alloc(spl_kmem_cache_t *skc, int flags, void **obj)
@@ -1143,48 +1189,49 @@ spl_emergency_alloc(spl_kmem_cache_t *skc, int flags, void **obj)
 		SRETURN(-ENOMEM);
 	}
 
-	if (skc->skc_ctor)
-		skc->skc_ctor(ske->ske_obj, skc->skc_private, flags);
-
 	spin_lock(&skc->skc_lock);
-	skc->skc_obj_total++;
-	skc->skc_obj_emergency++;
-	if (skc->skc_obj_emergency > skc->skc_obj_emergency_max)
-		skc->skc_obj_emergency_max = skc->skc_obj_emergency;
-
-	list_add(&ske->ske_list, &skc->skc_emergency_list);
+	empty = spl_emergency_insert(&skc->skc_emergency_tree, ske);
+	if (likely(empty)) {
+		skc->skc_obj_total++;
+		skc->skc_obj_emergency++;
+		if (skc->skc_obj_emergency > skc->skc_obj_emergency_max)
+			skc->skc_obj_emergency_max = skc->skc_obj_emergency;
+	}
 	spin_unlock(&skc->skc_lock);
 
+	if (unlikely(!empty)) {
+		kfree(ske->ske_obj);
+		kfree(ske);
+		SRETURN(-EINVAL);
+	}
+
+	if (skc->skc_ctor)
+		skc->skc_ctor(ske->ske_obj, skc->skc_private, flags);
+
 	*obj = ske->ske_obj;
 
 	SRETURN(0);
 }
 
 /*
- * Free the passed object if it is an emergency object or a normal slab
- * object.  Currently this is done by walking what should be a short list of
- * emergency objects.  If this proves to be too inefficient we can replace
- * the simple list with a hash.
+ * Locate the passed object in the red black tree and free it.
  */
 static int
 spl_emergency_free(spl_kmem_cache_t *skc, void *obj)
 {
-	spl_kmem_emergency_t *m, *n, *ske = NULL;
+	spl_kmem_emergency_t *ske;
 	SENTRY;
 
 	spin_lock(&skc->skc_lock);
-	list_for_each_entry_safe(m, n, &skc->skc_emergency_list, ske_list) {
-		if (m->ske_obj == obj) {
-			list_del(&m->ske_list);
-			skc->skc_obj_emergency--;
-			skc->skc_obj_total--;
-			ske = m;
-			break;
-		}
+	ske = spl_emergency_search(&skc->skc_emergency_tree, obj);
+	if (likely(ske)) {
+		rb_erase(&ske->ske_node, &skc->skc_emergency_tree);
+		skc->skc_obj_emergency--;
+		skc->skc_obj_total--;
 	}
 	spin_unlock(&skc->skc_lock);
 
-	if (ske == NULL)
+	if (unlikely(ske == NULL))
 		SRETURN(-ENOENT);
 
 	if (skc->skc_dtor)
@@ -1483,7 +1530,7 @@ spl_kmem_cache_create(char *name, size_t size, size_t align,
 	INIT_LIST_HEAD(&skc->skc_list);
 	INIT_LIST_HEAD(&skc->skc_complete_list);
 	INIT_LIST_HEAD(&skc->skc_partial_list);
-	INIT_LIST_HEAD(&skc->skc_emergency_list);
+	skc->skc_emergency_tree = RB_ROOT;
 	spin_lock_init(&skc->skc_lock);
 	init_waitqueue_head(&skc->skc_waitq);
 	skc->skc_slab_fail = 0;
@@ -1590,7 +1637,6 @@ spl_kmem_cache_destroy(spl_kmem_cache_t *skc)
 	ASSERT3U(skc->skc_obj_total, ==, 0);
 	ASSERT3U(skc->skc_obj_emergency, ==, 0);
 	ASSERT(list_empty(&skc->skc_complete_list));
-	ASSERT(list_empty(&skc->skc_emergency_list));
 
 	kmem_free(skc->skc_name, skc->skc_name_size);
 	spin_unlock(&skc->skc_lock);