Btfs: More metadata allocator optimizations

This lowers the empty cluster target for metadata allocations.  The lower
target makes it easier to do allocations and still seems to perform well.

It also fixes the allocator loop to drop the empty cluster when things
start getting difficult, avoiding false enospc warnings.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -2116,6 +2116,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 	struct btrfs_root * root = orig_root->fs_info->extent_root;
 	u64 total_needed = num_bytes;
 	u64 *last_ptr = NULL;
+	u64 last_wanted = 0;
 	struct btrfs_block_group_cache *block_group = NULL;
 	int chunk_alloc_done = 0;
 	int empty_cluster = 2 * 1024 * 1024;
@@ -2134,22 +2135,28 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 
 	if (data & BTRFS_BLOCK_GROUP_METADATA) {
 		last_ptr = &root->fs_info->last_alloc;
-		empty_cluster = 256 * 1024;
+		empty_cluster = 64 * 1024;
 	}
 
 	if ((data & BTRFS_BLOCK_GROUP_DATA) && btrfs_test_opt(root, SSD))
 		last_ptr = &root->fs_info->last_data_alloc;
 
 	if (last_ptr) {
-		if (*last_ptr)
+		if (*last_ptr) {
 			hint_byte = *last_ptr;
-		else
+			last_wanted = *last_ptr;
+		} else
 			empty_size += empty_cluster;
+	} else {
+		empty_cluster = 0;
 	}
 	search_start = max(search_start, first_logical_byte(root, 0));
 	search_start = max(search_start, hint_byte);
 	total_needed += empty_size;
 
+	if (search_start != last_wanted)
+		last_wanted = 0;
+
 	block_group = btrfs_lookup_block_group(root->fs_info, search_start);
 	if (!block_group)
 		block_group = btrfs_lookup_first_block_group(root->fs_info,
@@ -2195,9 +2202,9 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 			if (search_start + num_bytes > end)
 				goto new_group;
 
-			if (last_ptr && *last_ptr && search_start != *last_ptr) {
+			if (last_wanted && search_start != last_wanted) {
 				total_needed += empty_cluster;
-				*last_ptr = 0;
+				last_wanted = 0;
 				/*
 				 * if search_start is still in this block group
 				 * then we just re-search this block group
@@ -2223,6 +2230,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 				if (search_start >= start &&
 				    search_start < end) {
 					mutex_unlock(&block_group->alloc_mutex);
+					last_wanted = 0;
 					continue;
 				}
 
@@ -2240,6 +2248,11 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 			break;
 		}
 new_group:
+		last_wanted = 0;
+		if (loop > 0) {
+			total_needed -= empty_cluster;
+			empty_cluster = 0;
+		}
 		mutex_unlock(&block_group->alloc_mutex);
 		/*
 		 * Here's how this works.
@@ -2256,11 +2269,6 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 		if (loop == 0) {
 			head = &space_info->block_groups;
 			cur = head->next;
-
-			if (last_ptr && *last_ptr) {
-				total_needed += empty_cluster;
-				*last_ptr = 0;
-			}
 			loop++;
 		} else if (loop == 1 && cur == head) {
 			if (allowed_chunk_alloc && !chunk_alloc_done) {