[RFC PATCH-tip 6/6] xfs: Enable reader optimistic spinning for DAX inodes

[Waiman Long <Waiman.Long@xxxxxxx>]

This patch enables reader optimistic spinning for inodes that are
under a DAX-based mount point.

On a 4-socket Haswell machine running on a 4.7-rc1 tip-based kernel,
the fio test with multithreaded randrw and randwrite tests on the
same file on a XFS partition on top of a NVDIMM with DAX were run,
the aggregated bandwidths before and after the patch were as follows:

  Test      BW before patch     BW after patch  % change
  ----      ---------------     --------------  --------
  randrw        1352 MB/s          2164 MB/s      +60%
  randwrite     1710 MB/s          2550 MB/s      +49%

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 fs/xfs/xfs_icache.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index 99ee6ee..09f284f 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -71,6 +71,15 @@ xfs_inode_alloc(
 
        mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
 
+       /*
+        * Enable reader spinning for DAX nount point
+        */
+       if (mp->m_flags & XFS_MOUNT_DAX) {
+               rwsem_set_rspin_threshold(&ip->i_iolock.mr_lock);
+               rwsem_set_rspin_threshold(&ip->i_mmaplock.mr_lock);
+               rwsem_set_rspin_threshold(&ip->i_lock.mr_lock);
+       }
+
        /* initialise the xfs inode */
        ip->i_ino = ino;
        ip->i_mount = mp;
-- 
1.7.1