[PATCH] Avoid useless inodes and dentries reclamation

[Tim Chen <tim.c.chen@linux.intel.com>]

This patch detects that when free inodes and dentries are really
low, their reclamation is skipped so we do not have to contend
on the global sb_lock uselessly under memory pressure. Otherwise
we create a log jam trying to acquire the sb_lock in prune_super(),
with little or no freed memory to show for the effort.

The profile below shows a multi-threaded large file read exerting
pressure on memory with page cache usage.  It is dominated
by the sb_lock contention in the cpu cycles profile.  The patch
eliminates the sb_lock contention almost entirely for prune_super().

    43.94%           usemem  [kernel.kallsyms]             [k] _raw_spin_lock
                     |
                     --- _raw_spin_lock
                        |
                        |--32.44%-- grab_super_passive
                        |          prune_super
                        |          shrink_slab
                        |          do_try_to_free_pages
                        |          try_to_free_pages
                        |          __alloc_pages_nodemask
                        |          alloc_pages_current
                        |
                        |--32.18%-- put_super
                        |          drop_super
                        |          prune_super
                        |          shrink_slab
                        |          do_try_to_free_pages
                        |          try_to_free_pages
                        |          __alloc_pages_nodemask
                        |          alloc_pages_current

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 fs/super.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 68307c0..70fa26c 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -53,6 +53,7 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
  * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
  * take a passive reference to the superblock to avoid this from occurring.
  */
+#define SB_CACHE_LOW 5
 static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 {
 	struct super_block *sb;
@@ -68,6 +69,13 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 	if (sc->nr_to_scan && !(sc->gfp_mask & __GFP_FS))
 		return -1;
 
+	/*
+	 * Don't prune if we have few cached objects to reclaim to
+	 * avoid useless sb_lock contention
+	 */
+	if ((sb->s_nr_dentry_unused + sb->s_nr_inodes_unused) <= SB_CACHE_LOW)
+		return -1;
+
 	if (!grab_super_passive(sb))
 		return -1;
 
-- 
1.7.11.7