[PATCH][BUGFIX][RFC] fix soft lock up at NFS mount by making limitation of dentry_unused

[Kentaro Makita <k-makita@np.css.fujitsu.com>]

[Summary]
  Make a limitation of dentry_unused to avoid soft lock up at NFS mounts
 and remounting any filesystem.

[Descriptions]
- background
 dentry_unused is a list of dentries which is not in use. This works
 as a cache against not-exisiting files. dentry_unused grows up when
 directories or files are removed. This list can be *verrry* long
 if there is no memory  pressure, because there is no limit.

- what's problem
 When prune_dcache() is called, it scans *all* dentry_unused linearly
 under spin_lock(). This scan costs very much if there are many entries.
 For example, prune_dcache() is called at mounting NFS.
 In our test, when there are 100,000,000 of unused dentries, mounting
 NFS took 1 minutes and almost all user programs hang during it.

  100,000,000 is possible number on large systems.

 This problem already happend on our system.
 Therefore, we need a limitation of dentry_unused.

- How to fix
 Limit number of unused dentries to suitable value.

 Threshold is as follows:
 dentry_unused_ratio: default value is 10000(%). If the amount of
 dentry_unused reaches to 10000% of the amount of dentry_in_use,
 5% of them are freed.

 I feel we need more tests to determine resonable value to any system.
 So, please test.

 This patch is based on linux-2.6.25-rc4.

-Test Results

 Result on 24GB boxes with excessive unused dentries.
Without patch:
# cat /proc/sys/fs/dentry-state
103327453       103313783       45      0       0
# time mount -t nfs 192.168.0.2:/export /mnt
real    1m4.698s
user    0m0.000s
sys     1m4.672s

 With this patch:
# cat /proc/sys/fs/dentry-state
118681  117225  45      0       0       0
# time mount -t nfs 192.168.0.2:/export /mnt
real    0m0.103s
user    0m0.004s
sys     0m0.076s

Tested on Intel Itanium 2 9050 (dualcore) x12 MEM 24GB , kernel-2.6.25-rc4
I found no peformance regression in my tests.


Best Regards,
Kentaro Makita

Signed-off-by: Kentaro Makita <k-makita@np.css.fujitsu.com>
---
 fs/dcache.c |    7 +++++++
 1 files changed, 7 insertions(+)
diff -rupN -X linux-2.6.25-rc4/Documentation/dontdiff linux-2.6.25-rc4/fs/dcache.c linux-2.6.25-rc4mod/fs/dcache.c
--- linux-2.6.25-rc4/fs/dcache.c	2008-03-05 13:33:54.000000000 +0900
+++ linux-2.6.25-rc4mod/fs/dcache.c	2008-03-05 16:47:18.000000000 +0900
@@ -42,6 +42,8 @@ __cacheline_aligned_in_smp DEFINE_SEQLOC

 EXPORT_SYMBOL(dcache_lock);

+/* threshold to limit dentry_unused */
+unsigned int dentry_unused_ratio = 10000;
 static struct kmem_cache *dentry_cache __read_mostly;

 #define DNAME_INLINE_LEN (sizeof(struct dentry)-offsetof(struct dentry,d_iname))
@@ -61,6 +63,7 @@ static unsigned int d_hash_mask __read_m
 static unsigned int d_hash_shift __read_mostly;
 static struct hlist_head *dentry_hashtable __read_mostly;
 static LIST_HEAD(dentry_unused);
+static void prune_dcache(int count, struct super_block *sb);

 /* Statistics gathering. */
 struct dentry_stat_t dentry_stat = {
@@ -214,6 +217,10 @@ repeat:
   	}
  	spin_unlock(&dentry->d_lock);
 	spin_unlock(&dcache_lock);
+	/* Prune unused dentry over threshold level */
+	int nr_in_use = (dentry_stat.nr_dentry - dentry_stat.nr_unused);
+	if (dentry_stat.nr_dentry > nr_in_use * dentry_unused_ratio / 100)
+		prune_dcache(dentry_stat.nr_unused * 5 / 100 , NULL);
 	return;

 unhash_it: