[PATCH] struct dentry: place d_hash close to d_parent and d_name to speedup lookups

dentry cache uses sophisticated RCU technology (and prefetching if
available) but touches 2 cache lines per dentry during hlist lookup.

This patch moves d_hash in the same cache line than d_parent and d_name
fields so that :

1) One cache line is needed instead of two.

2) the hlist_for_each_rcu() prefetching has a chance to bring all the
   needed data in advance, not only the part that includes d_hash.next.

I also changed one old comment that was wrong for 64bits.

A further optimisation would be to separate dentry in two parts, one that
is mostly read, and one writen (d_count/d_lock) to avoid false sharing on
SMP/NUMA but this would need different field placement depending on 32bits
or 64bits platform.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/dcache.h

@@ -88,8 +88,9 @@ struct dentry {
 					 * negative */
 	/*
 	 * The next three fields are touched by __d_lookup.  Place them here
-	 * so they all fit in a 16-byte range, with 16-byte alignment.
+	 * so they all fit in a cache line.
 	 */
+	struct hlist_node d_hash;	/* lookup hash list */
 	struct dentry *d_parent;	/* parent directory */
 	struct qstr d_name;
 
@@ -103,7 +104,6 @@ struct dentry {
 	void *d_fsdata;			/* fs-specific data */
  	struct rcu_head d_rcu;
 	struct dcookie_struct *d_cookie; /* cookie, if any */
-	struct hlist_node d_hash;	/* lookup hash list */	
 	int d_mounted;
 	unsigned char d_iname[DNAME_INLINE_LEN_MIN];	/* small names */
 };