linux-kernel - [PATCH] Avoid useless inodes and dentries reclamation

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Date:	Wed, 28 Aug 2013 14:52:12 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Dave Chinner <dchinner@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH] Avoid useless inodes and dentries reclamation

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@...ux.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


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