Btrfs: run the backing dev more often in the submit_bio helper

The submit_bio helper thread can decide to loop back around to
service more bios.  This commit forces it to unplug first, which helps
reduce the latency seen by submitters.

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

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 41ecbb2..ace2e8d 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -325,16 +325,6 @@ loop_lock:
                num_sync_run = 0;
                blk_run_backing_dev(bdi, NULL);
        }
-
-       cond_resched();
-       if (again)
-               goto loop;
-
-       spin_lock(&device->io_lock);
-       if (device->pending_bios.head || device->pending_sync_bios.head)
-               goto loop_lock;
-       spin_unlock(&device->io_lock);
-
        /*
         * IO has already been through a long path to get here.  Checksumming,
         * async helper threads, perhaps compression.  We've done a pretty
@@ -346,6 +336,16 @@ loop_lock:
         * cared about found its way down here.
         */
        blk_run_backing_dev(bdi, NULL);
+
+       cond_resched();
+       if (again)
+               goto loop;
+
+       spin_lock(&device->io_lock);
+       if (device->pending_bios.head || device->pending_sync_bios.head)
+               goto loop_lock;
+       spin_unlock(&device->io_lock);
+
 done:
        return 0;
 }