[RFC PATCH 2/2] xfs: Allow degeneration of m_fdblocks/m_ifree to global counters

[Waiman Long]

Small XFS filesystems on systems with large number of CPUs can incur a
significant overhead due to excessive calls to the percpu_counter_sum()
function which needs to walk through a large number of different
cachelines.

This patch uses the newly added percpu_counter_set_limit() API to
potentially switch the m_fdblocks and m_ifree per-cpu counters to
a global counter with locks at filesystem mount time if its size
is small relatively to the number of CPUs available.

A possible use case is the use of the NVDIMM as an application scratch
storage area for log file and other small files. Current battery-backed
NVDIMMs are pretty small in size, e.g. 8G per DIMM. So we cannot create
large filesystem on top of them.

On a 4-socket 80-thread system running 4.5-rc6 kernel, this patch can
improve the throughput of the AIM7 XFS disk workload by 25%. Before
the patch, the perf profile was:

  18.68%   0.08%  reaim  [k] __percpu_counter_compare
  18.05%   9.11%  reaim  [k] __percpu_counter_sum
   0.37%   0.36%  reaim  [k] __percpu_counter_add

After the patch, the perf profile was:

   0.73%   0.36%  reaim  [k] __percpu_counter_add
   0.27%   0.27%  reaim  [k] __percpu_counter_compare

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 fs/xfs/xfs_mount.c |    1 -
 fs/xfs/xfs_mount.h |    5 +++++
 fs/xfs/xfs_super.c |    6 ++++++
 3 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index bb753b3..fe74b91 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1163,7 +1163,6 @@ xfs_mod_ifree(
  * a large batch count (1024) to minimise global counter updates except when
  * we get near to ENOSPC and we have to be very accurate with our updates.
  */
-#define XFS_FDBLOCKS_BATCH	1024
 int
 xfs_mod_fdblocks(
 	struct xfs_mount	*mp,
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index b570984..d9520f4 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -206,6 +206,11 @@ typedef struct xfs_mount {
 #define	XFS_WSYNC_WRITEIO_LOG	14	/* 16k */
 
 /*
+ * FD blocks batch size for per-cpu compare
+ */
+#define XFS_FDBLOCKS_BATCH	1024
+
+/*
  * Allow large block sizes to be reported to userspace programs if the
  * "largeio" mount option is used.
  *
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 59c9b7b..c0b4f79 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1412,6 +1412,12 @@ xfs_reinit_percpu_counters(
 	percpu_counter_set(&mp->m_icount, mp->m_sb.sb_icount);
 	percpu_counter_set(&mp->m_ifree, mp->m_sb.sb_ifree);
 	percpu_counter_set(&mp->m_fdblocks, mp->m_sb.sb_fdblocks);
+
+	/*
+	 * Use default batch size for m_ifree
+	 */
+	percpu_counter_set_limit(&mp->m_ifree, 0);
+	percpu_counter_set_limit(&mp->m_fdblocks, 4 * XFS_FDBLOCKS_BATCH);
 }
 
 static void
-- 
1.7.1