[PATCH] mbcache: Speed up cache entry creation

[Sultan Alsawaf <sultan@kerneltoast.com>]

From: Sultan Alsawaf <sultan@kerneltoast.com>

In order to prevent redundant entry creation by racing against itself,
mb_cache_entry_create scans through a hash-list of all current entries
in order to see if another allocation for the requested new entry has
been made. Furthermore, it allocates memory for a new entry before
scanning through this hash-list, which results in that allocated memory
being discarded when the requested new entry is already present.

Speed up cache entry creation by keeping a small linked list of
requested new entries in progress, and scanning through that first
instead of the large hash-list. And don't allocate memory for a new
entry until it's known that the allocated memory will be used.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 fs/mbcache.c | 82 ++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 57 insertions(+), 25 deletions(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index 97c54d3a2227..289f3664061e 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -25,9 +25,14 @@
  * size hash table is used for fast key lookups.
  */
 
+struct mb_bucket {
+	struct hlist_bl_head hash;
+	struct list_head req_list;
+};
+
 struct mb_cache {
 	/* Hash table of entries */
-	struct hlist_bl_head	*c_hash;
+	struct mb_bucket	*c_bucket;
 	/* log2 of hash table size */
 	int			c_bucket_bits;
 	/* Maximum entries in cache to avoid degrading hash too much */
@@ -42,15 +47,21 @@ struct mb_cache {
 	struct work_struct	c_shrink_work;
 };
 
+struct mb_cache_req {
+	struct list_head lnode;
+	u32 key;
+	u64 value;
+};
+
 static struct kmem_cache *mb_entry_cache;
 
 static unsigned long mb_cache_shrink(struct mb_cache *cache,
 				     unsigned long nr_to_scan);
 
-static inline struct hlist_bl_head *mb_cache_entry_head(struct mb_cache *cache,
-							u32 key)
+static inline struct mb_bucket *mb_cache_entry_bucket(struct mb_cache *cache,
+						      u32 key)
 {
-	return &cache->c_hash[hash_32(key, cache->c_bucket_bits)];
+	return &cache->c_bucket[hash_32(key, cache->c_bucket_bits)];
 }
 
 /*
@@ -77,6 +88,8 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 	struct mb_cache_entry *entry, *dup;
 	struct hlist_bl_node *dup_node;
 	struct hlist_bl_head *head;
+	struct mb_cache_req *tmp_req, req;
+	struct mb_bucket *bucket;
 
 	/* Schedule background reclaim if there are too many entries */
 	if (cache->c_entry_count >= cache->c_max_entries)
@@ -85,9 +98,33 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 	if (cache->c_entry_count >= 2*cache->c_max_entries)
 		mb_cache_shrink(cache, SYNC_SHRINK_BATCH);
 
+	bucket = mb_cache_entry_bucket(cache, key);
+	head = &bucket->hash;
+	hlist_bl_lock(head);
+	list_for_each_entry(tmp_req, &bucket->req_list, lnode) {
+		if (tmp_req->key == key && tmp_req->value == value) {
+			hlist_bl_unlock(head);
+			return -EBUSY;
+		}
+	}
+	hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
+		if (dup->e_key == key && dup->e_value == value) {
+			hlist_bl_unlock(head);
+			return -EBUSY;
+		}
+	}
+	req.key = key;
+	req.value = value;
+	list_add(&req.lnode, &bucket->req_list);
+	hlist_bl_unlock(head);
+
 	entry = kmem_cache_alloc(mb_entry_cache, mask);
-	if (!entry)
+	if (!entry) {
+		hlist_bl_lock(head);
+		list_del(&req.lnode);
+		hlist_bl_unlock(head);
 		return -ENOMEM;
+	}
 
 	INIT_LIST_HEAD(&entry->e_list);
 	/* One ref for hash, one ref returned */
@@ -96,15 +133,9 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 	entry->e_value = value;
 	entry->e_reusable = reusable;
 	entry->e_referenced = 0;
-	head = mb_cache_entry_head(cache, key);
+
 	hlist_bl_lock(head);
-	hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
-		if (dup->e_key == key && dup->e_value == value) {
-			hlist_bl_unlock(head);
-			kmem_cache_free(mb_entry_cache, entry);
-			return -EBUSY;
-		}
-	}
+	list_del(&req.lnode);
 	hlist_bl_add_head(&entry->e_hash_list, head);
 	hlist_bl_unlock(head);
 
@@ -133,7 +164,7 @@ static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 	struct hlist_bl_node *node;
 	struct hlist_bl_head *head;
 
-	head = mb_cache_entry_head(cache, key);
+	head = &mb_cache_entry_bucket(cache, key)->hash;
 	hlist_bl_lock(head);
 	if (entry && !hlist_bl_unhashed(&entry->e_hash_list))
 		node = entry->e_hash_list.next;
@@ -202,7 +233,7 @@ struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 	struct hlist_bl_head *head;
 	struct mb_cache_entry *entry;
 
-	head = mb_cache_entry_head(cache, key);
+	head = &mb_cache_entry_bucket(cache, key)->hash;
 	hlist_bl_lock(head);
 	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 		if (entry->e_key == key && entry->e_value == value) {
@@ -230,7 +261,7 @@ void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 	struct hlist_bl_head *head;
 	struct mb_cache_entry *entry;
 
-	head = mb_cache_entry_head(cache, key);
+	head = &mb_cache_entry_bucket(cache, key)->hash;
 	hlist_bl_lock(head);
 	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
 		if (entry->e_key == key && entry->e_value == value) {
@@ -300,7 +331,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 		 * from under us.
 		 */
 		spin_unlock(&cache->c_list_lock);
-		head = mb_cache_entry_head(cache, entry->e_key);
+		head = &mb_cache_entry_bucket(cache, entry->e_key)->hash;
 		hlist_bl_lock(head);
 		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 			hlist_bl_del_init(&entry->e_hash_list);
@@ -354,21 +385,22 @@ struct mb_cache *mb_cache_create(int bucket_bits)
 	cache->c_max_entries = bucket_count << 4;
 	INIT_LIST_HEAD(&cache->c_list);
 	spin_lock_init(&cache->c_list_lock);
-	cache->c_hash = kmalloc_array(bucket_count,
-				      sizeof(struct hlist_bl_head),
-				      GFP_KERNEL);
-	if (!cache->c_hash) {
+	cache->c_bucket = kmalloc_array(bucket_count, sizeof(*cache->c_bucket),
+					GFP_KERNEL);
+	if (!cache->c_bucket) {
 		kfree(cache);
 		goto err_out;
 	}
-	for (i = 0; i < bucket_count; i++)
-		INIT_HLIST_BL_HEAD(&cache->c_hash[i]);
+	for (i = 0; i < bucket_count; i++) {
+		INIT_HLIST_BL_HEAD(&cache->c_bucket[i].hash);
+		INIT_LIST_HEAD(&cache->c_bucket[i].req_list);
+	}
 
 	cache->c_shrink.count_objects = mb_cache_count;
 	cache->c_shrink.scan_objects = mb_cache_scan;
 	cache->c_shrink.seeks = DEFAULT_SEEKS;
 	if (register_shrinker(&cache->c_shrink)) {
-		kfree(cache->c_hash);
+		kfree(cache->c_bucket);
 		kfree(cache);
 		goto err_out;
 	}
@@ -409,7 +441,7 @@ void mb_cache_destroy(struct mb_cache *cache)
 		WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 		mb_cache_entry_put(cache, entry);
 	}
-	kfree(cache->c_hash);
+	kfree(cache->c_bucket);
 	kfree(cache);
 }
 EXPORT_SYMBOL(mb_cache_destroy);
-- 
2.22.0