slob: reduce external fragmentation by using three free lists

By putting smaller objects on their own list, we greatly reduce overall
external fragmentation and increase repeatability.  This reduces total SLOB
overhead from > 50% to ~6% on a simple boot test.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slob.c

@@ -12,10 +12,17 @@
  * allocator is as little as 2 bytes, however typically most architectures
  * will require 4 bytes on 32-bit and 8 bytes on 64-bit.
  *
- * The slob heap is a linked list of pages from alloc_pages(), and
- * within each page, there is a singly-linked list of free blocks (slob_t).
- * The heap is grown on demand and allocation from the heap is currently
- * first-fit.
+ * The slob heap is a set of linked list of pages from alloc_pages(),
+ * and within each page, there is a singly-linked list of free blocks
+ * (slob_t). The heap is grown on demand. To reduce fragmentation,
+ * heap pages are segregated into three lists, with objects less than
+ * 256 bytes, objects less than 1024 bytes, and all other objects.
+ *
+ * Allocation from heap involves first searching for a page with
+ * sufficient free blocks (using a next-fit-like approach) followed by
+ * a first-fit scan of the page. Deallocation inserts objects back
+ * into the free list in address order, so this is effectively an
+ * address-ordered first fit.
  *
  * Above this is an implementation of kmalloc/kfree. Blocks returned
  * from kmalloc are prepended with a 4-byte header with the kmalloc size.
@@ -110,9 +117,13 @@ static inline void free_slob_page(struct slob_page *sp)
 }
 
 /*
- * All (partially) free slob pages go on this list.
+ * All partially free slob pages go on these lists.
  */
-static LIST_HEAD(free_slob_pages);
+#define SLOB_BREAK1 256
+#define SLOB_BREAK2 1024
+static LIST_HEAD(free_slob_small);
+static LIST_HEAD(free_slob_medium);
+static LIST_HEAD(free_slob_large);
 
 /*
  * slob_page: True for all slob pages (false for bigblock pages)
@@ -140,9 +151,9 @@ static inline int slob_page_free(struct slob_page *sp)
 	return test_bit(PG_private, &sp->flags);
 }
 
-static inline void set_slob_page_free(struct slob_page *sp)
+static void set_slob_page_free(struct slob_page *sp, struct list_head *list)
 {
-	list_add(&sp->list, &free_slob_pages);
+	list_add(&sp->list, list);
 	__set_bit(PG_private, &sp->flags);
 }
 
@@ -294,12 +305,20 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
 {
 	struct slob_page *sp;
 	struct list_head *prev;
+	struct list_head *slob_list;
 	slob_t *b = NULL;
 	unsigned long flags;
 
+	if (size < SLOB_BREAK1)
+		slob_list = &free_slob_small;
+	else if (size < SLOB_BREAK2)
+		slob_list = &free_slob_medium;
+	else
+		slob_list = &free_slob_large;
+
 	spin_lock_irqsave(&slob_lock, flags);
 	/* Iterate through each partially free page, try to find room */
-	list_for_each_entry(sp, &free_slob_pages, list) {
+	list_for_each_entry(sp, slob_list, list) {
 #ifdef CONFIG_NUMA
 		/*
 		 * If there's a node specification, search for a partial
@@ -321,9 +340,9 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
 		/* Improve fragment distribution and reduce our average
 		 * search time by starting our next search here. (see
 		 * Knuth vol 1, sec 2.5, pg 449) */
-		if (prev != free_slob_pages.prev &&
-				free_slob_pages.next != prev->next)
-			list_move_tail(&free_slob_pages, prev->next);
+		if (prev != slob_list->prev &&
+				slob_list->next != prev->next)
+			list_move_tail(slob_list, prev->next);
 		break;
 	}
 	spin_unlock_irqrestore(&slob_lock, flags);
@@ -341,7 +360,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
 		sp->free = b;
 		INIT_LIST_HEAD(&sp->list);
 		set_slob(b, SLOB_UNITS(PAGE_SIZE), b + SLOB_UNITS(PAGE_SIZE));
-		set_slob_page_free(sp);
+		set_slob_page_free(sp, slob_list);
 		b = slob_page_alloc(sp, size, align);
 		BUG_ON(!b);
 		spin_unlock_irqrestore(&slob_lock, flags);
@@ -387,7 +406,7 @@ static void slob_free(void *block, int size)
 		set_slob(b, units,
 			(void *)((unsigned long)(b +
 					SLOB_UNITS(PAGE_SIZE)) & PAGE_MASK));
-		set_slob_page_free(sp);
+		set_slob_page_free(sp, &free_slob_small);
 		goto out;
 	}