dm thin: sort the deferred cells

Sort the cells in logical block order before processing each cell in
process_thin_deferred_cells().  This significantly improves the ondisk
layout on rotational storage, whereby improving read performance.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin.c

@@ -17,6 +17,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/slab.h>
+#include <linux/sort.h>
 #include <linux/rbtree.h>
 
 #define	DM_MSG_PREFIX	"thin"
@@ -205,6 +206,8 @@ typedef void (*process_bio_fn)(struct thin_c *tc, struct bio *bio);
 typedef void (*process_cell_fn)(struct thin_c *tc, struct dm_bio_prison_cell *cell);
 typedef void (*process_mapping_fn)(struct dm_thin_new_mapping *m);
 
+#define CELL_SORT_ARRAY_SIZE 8192
+
 struct pool {
 	struct list_head list;
 	struct dm_target *ti;	/* Only set if a pool target is bound */
@@ -252,6 +255,8 @@ struct pool {
 
 	process_mapping_fn process_prepared_mapping;
 	process_mapping_fn process_prepared_discard;
+
+	struct dm_bio_prison_cell *cell_sort_array[CELL_SORT_ARRAY_SIZE];
 };
 
 static enum pool_mode get_pool_mode(struct pool *pool);
@@ -1800,12 +1805,48 @@ static void process_thin_deferred_bios(struct thin_c *tc)
 	blk_finish_plug(&plug);
 }
 
+static int cmp_cells(const void *lhs, const void *rhs)
+{
+	struct dm_bio_prison_cell *lhs_cell = *((struct dm_bio_prison_cell **) lhs);
+	struct dm_bio_prison_cell *rhs_cell = *((struct dm_bio_prison_cell **) rhs);
+
+	BUG_ON(!lhs_cell->holder);
+	BUG_ON(!rhs_cell->holder);
+
+	if (lhs_cell->holder->bi_iter.bi_sector < rhs_cell->holder->bi_iter.bi_sector)
+		return -1;
+
+	if (lhs_cell->holder->bi_iter.bi_sector > rhs_cell->holder->bi_iter.bi_sector)
+		return 1;
+
+	return 0;
+}
+
+static unsigned sort_cells(struct pool *pool, struct list_head *cells)
+{
+	unsigned count = 0;
+	struct dm_bio_prison_cell *cell, *tmp;
+
+	list_for_each_entry_safe(cell, tmp, cells, user_list) {
+		if (count >= CELL_SORT_ARRAY_SIZE)
+			break;
+
+		pool->cell_sort_array[count++] = cell;
+		list_del(&cell->user_list);
+	}
+
+	sort(pool->cell_sort_array, count, sizeof(cell), cmp_cells, NULL);
+
+	return count;
+}
+
 static void process_thin_deferred_cells(struct thin_c *tc)
 {
 	struct pool *pool = tc->pool;
 	unsigned long flags;
 	struct list_head cells;
-	struct dm_bio_prison_cell *cell, *tmp;
+	struct dm_bio_prison_cell *cell;
+	unsigned i, j, count;
 
 	INIT_LIST_HEAD(&cells);
 
@@ -1816,7 +1857,11 @@ static void process_thin_deferred_cells(struct thin_c *tc)
 	if (list_empty(&cells))
 		return;
 
-	list_for_each_entry_safe(cell, tmp, &cells, user_list) {
+	do {
+		count = sort_cells(tc->pool, &cells);
+
+		for (i = 0; i < count; i++) {
+			cell = pool->cell_sort_array[i];
 			BUG_ON(!cell->holder);
 
 			/*
@@ -1825,11 +1870,13 @@ static void process_thin_deferred_cells(struct thin_c *tc)
 			 * prepared mappings to process.
 			 */
 			if (ensure_next_mapping(pool)) {
+				for (j = i; j < count; j++)
+					list_add(&pool->cell_sort_array[j]->user_list, &cells);
+
 				spin_lock_irqsave(&tc->lock, flags);
-			list_add(&cell->user_list, &tc->deferred_cells);
 				list_splice(&cells, &tc->deferred_cells);
 				spin_unlock_irqrestore(&tc->lock, flags);
-			break;
+				return;
 			}
 
 			if (cell->holder->bi_rw & REQ_DISCARD)
@@ -1837,6 +1884,7 @@ static void process_thin_deferred_cells(struct thin_c *tc)
 			else
 				pool->process_cell(tc, cell);
 		}
+	} while (!list_empty(&cells));
 }
 
 static void thin_get(struct thin_c *tc);