Btrfs: reduce stalls during transaction commit

To avoid deadlocks and reduce latencies during some critical operations, some
transaction writers are allowed to jump into the running transaction and make
it run a little longer, while others sit around and wait for the commit to
finish.

This is a bit unfair, especially when the callers that jump in do a bunch
of IO that makes all the others procs on the box wait.  This commit
reduces the stalls this produces by pre-reading file extent pointers
during btrfs_finish_ordered_io before the transaction is joined.

It also tunes the drop_snapshot code to politely wait for transactions
that have started writing out their delayed refs to finish.  This avoids
new delayed refs being flooded into the queue while we're trying to
close off the transaction.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 7d4f948bc22a..13a17477c4f4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1502,6 +1502,7 @@ static int btrfs_finish_ordered_io(struct inode *inode, u64 start, u64 end)
         struct btrfs_trans_handle *trans;
         struct btrfs_ordered_extent *ordered_extent;
         struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
+        struct btrfs_path *path;
         int compressed = 0;
         int ret;
 
@@ -1509,6 +1510,23 @@ static int btrfs_finish_ordered_io(struct inode *inode, u64 start, u64 end)
         if (!ret)
                 return 0;
 
+        /*
+         * before we join the transaction, try to do some of our IO.
+         * This will limit the amount of IO that we have to do with
+         * the transaction running.  We're unlikely to need to do any
+         * IO if the file extents are new, the disk_i_size checks
+         * covers the most common case.
+         */
+        if (start < BTRFS_I(inode)->disk_i_size) {
+                path = btrfs_alloc_path();
+                if (path) {
+                        ret = btrfs_lookup_file_extent(NULL, root, path,
+                                                       inode->i_ino,
+                                                       start, 0);
+                        btrfs_free_path(path);
+                }
+        }
+
         trans = btrfs_join_transaction(root, 1);
 
         ordered_extent = btrfs_lookup_ordered_extent(inode, start);