Btrfs: streamline tree-log btree block writeout

Syncing the tree log is a 3 phase operation.

1) write and wait for all the tree log blocks for a given root.

2) write and wait for all the tree log blocks for the
tree of tree log roots.

3) write and wait for the super blocks (barriers here)

This isn't as efficient as it could be because there is
no requirement to wait for the blocks from step one to hit the disk
before we start writing the blocks from step two.  This commit
changes the sequence so that we don't start waiting until
all the tree blocks from both steps one and two have been sent
to disk.

We do this by breaking up btrfs_write_wait_marked_extents into
two functions, which is trivial because it was already broken
up into two parts.

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

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 0a1bde268963..4aff766d171a 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2013,7 +2013,10 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
                 goto out;
         }
 
-        ret = btrfs_write_and_wait_marked_extents(log, &log->dirty_log_pages);
+        /* we start IO on  all the marked extents here, but we don't actually
+         * wait for them until later.
+         */
+        ret = btrfs_write_marked_extents(log, &log->dirty_log_pages);
         BUG_ON(ret);
 
         btrfs_set_root_node(&log->root_item, log->node);
@@ -2048,6 +2051,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 
         index2 = log_root_tree->log_transid % 2;
         if (atomic_read(&log_root_tree->log_commit[index2])) {
+                btrfs_wait_marked_extents(log, &log->dirty_log_pages);
                 wait_log_commit(trans, log_root_tree,
                                 log_root_tree->log_transid);
                 mutex_unlock(&log_root_tree->log_mutex);
@@ -2067,6 +2071,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
          * check the full commit flag again
          */
         if (root->fs_info->last_trans_log_full_commit == trans->transid) {
+                btrfs_wait_marked_extents(log, &log->dirty_log_pages);
                 mutex_unlock(&log_root_tree->log_mutex);
                 ret = -EAGAIN;
                 goto out_wake_log_root;
@@ -2075,6 +2080,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
         ret = btrfs_write_and_wait_marked_extents(log_root_tree,
                                 &log_root_tree->dirty_log_pages);
         BUG_ON(ret);
+        btrfs_wait_marked_extents(log, &log->dirty_log_pages);
 
         btrfs_set_super_log_root(&root->fs_info->super_for_commit,
                                 log_root_tree->node->start);