[PATCH v2] btrfs: speedup mount time with readahead chunk tree

[robbieko <robbieko@synology.com>]

From: Robbie Ko <robbieko@synology.com>

When mounting, we always need to read the whole chunk tree,
when there are too many chunk items, most of the time is
spent on btrfs_read_chunk_tree, because we only read one
leaf at a time.

It is unreasonable to limit the readahead mechanism to a
range of 64k, so we have removed that limit.

In addition we added reada_maximum_size to customize the
size of the pre-reader, The default is 64k to maintain the
original behavior.

So we fix this by used readahead mechanism, and set readahead
max size to ULLONG_MAX which reads all the leaves after the
key in the node when reading a level 1 node.

I have a test environment as follows:

200TB btrfs volume: used 192TB

Data, single: total=192.00TiB, used=192.00TiB
System, DUP: total=40.00MiB, used=19.91MiB
Metadata, DUP: total=63.00GiB, used=46.46GiB
GlobalReserve, single: total=2.00GiB, used=0.00B

chunk tree level : 2
chunk tree tree:
   nodes: 4
   leaves: 1270
   total: 1274
chunk tree size: 19.9 MB
SYSTEM chunks count : 2 (8MB, 32MB)

btrfs_read_chunk_tree spends the following time:
before: 1.89s
patch: 0.27s
Speed increase of about 85%.

Signed-off-by: Robbie Ko <robbieko@synology.com>
---
Changelog:
v2:
- add performance testing
- remove readahead logical bytenr 64k limit
- add reada_maximum_size for customize
---
 fs/btrfs/ctree.c   | 23 +++++++++++------------
 fs/btrfs/ctree.h   |  1 +
 fs/btrfs/volumes.c |  2 ++
 3 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 3a7648bff42c..dc84f526cd93 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -75,7 +75,14 @@ size_t __const btrfs_get_num_csums(void)
 
 struct btrfs_path *btrfs_alloc_path(void)
 {
-	return kmem_cache_zalloc(btrfs_path_cachep, GFP_NOFS);
+	struct btrfs_path *path;
+
+	path = kmem_cache_zalloc(btrfs_path_cachep, GFP_NOFS);
+
+	if (path)
+		path->reada_maximum_size = 65536;
+
+	return path;
 }
 
 /* this also releases the path */
@@ -2161,12 +2168,10 @@ static void reada_for_search(struct btrfs_fs_info *fs_info,
 	struct btrfs_disk_key disk_key;
 	u32 nritems;
 	u64 search;
-	u64 target;
 	u64 nread = 0;
 	struct extent_buffer *eb;
 	u32 nr;
 	u32 blocksize;
-	u32 nscan = 0;
 
 	if (level != 1)
 		return;
@@ -2184,8 +2189,6 @@ static void reada_for_search(struct btrfs_fs_info *fs_info,
 		return;
 	}
 
-	target = search;
-
 	nritems = btrfs_header_nritems(node);
 	nr = slot;
 
@@ -2205,13 +2208,9 @@ static void reada_for_search(struct btrfs_fs_info *fs_info,
 				break;
 		}
 		search = btrfs_node_blockptr(node, nr);
-		if ((search <= target && target - search <= 65536) ||
-		    (search > target && search - target <= 65536)) {
-			readahead_tree_block(fs_info, search);
-			nread += blocksize;
-		}
-		nscan++;
-		if ((nread > 65536 || nscan > 32))
+		readahead_tree_block(fs_info, search);
+		nread += blocksize;
+		if (nread > path->reada_maximum_size)
 			break;
 	}
 }
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index d404cce8ae40..ea88a6473eb8 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -360,6 +360,7 @@ struct btrfs_path {
 	/* if there is real range locking, this locks field will change */
 	u8 locks[BTRFS_MAX_LEVEL];
 	u8 reada;
+	u64 reada_maximum_size;
 	/* keep some upper locks as we walk down */
 	u8 lowest_level;
 
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0d6e785bcb98..fc87b2e9e865 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7043,6 +7043,8 @@ int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
+	path->reada = READA_FORWARD;
+	path->reada_maximum_size = ULLONG_MAX;
 
 	/*
 	 * uuid_mutex is needed only if we are mounting a sprout FS
-- 
2.17.1