Re: [PATCH][BUGFIX][RFC] fix soft lock up at NFS mount by makinglimitation of dentry_unused

[Kentaro Makita]

David Chinner wrote:
> On Thu, Mar 06, 2008 at 01:41:29PM +0900, Kentaro Makita wrote:
....
>>   100,000,000 is possible number on large systems.
>>
>>  This problem already happend on our system.
>>  Therefore, we need a limitation of dentry_unused.
> 
> No, we need a smarter free list structure. There have been several attempts
> at this in the past. Two that I can recall off the top of my head:
> 
> 	- per node unused LRUs
> 	- per superblock unusued LRUs

 I know there is such attempt already, but they are not in main-line.
 I think this is not a smart but simple way to avoid this ploblem.
> 
> I guess we need to revisit this again, because limiting the size of
> the cache like this is not an option.
> 
>>  I feel we need more tests to determine resonable value to any system.
>>  So, please test.
> .....
>> Tested on Intel Itanium 2 9050 (dualcore) x12 MEM 24GB , kernel-2.6.25-rc4
>> I found no peformance regression in my tests.
> 
> Try something that relies on leaving the working set on the unused
> list, like NFS server benchmarks that have a working set of tens of
> million of files....
> 
 Okay, I'll try some benchmarks and report results...
>> Signed-off-by: Kentaro Makita <k-makita@xxxxxxxxxxxxxxxxxx>
>> ---
>>  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
......
>> @@ -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);
> 
> nr_in_use is going to overflow 32 bits with this calculation.
 Oh, I simply mistake. I fix it at this post.
> 
> Cheers,
> 
> Dave.
Best Regards,
Kentaro Makita

Signed-off-by: Kentaro Makita <k-makita@xxxxxxxxxxxxxxxxxx>
---
 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	2008-03-06 15:27:22.000000000 +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;

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