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@xxxxxxxxxxxxxxx>
---
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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