netdev - [PATCH] net: fold network name hash

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Date:	Tue, 27 Oct 2009 10:22:51 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	eric.dumazet@...il.com, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, opurdila@...acom.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk
Subject: [PATCH] net: fold network name hash

The full_name_hash does not produce a value that is evenly distributed
over the lower 8 bits. This causes name hash to be unbalanced with large
number of names.  A simple fix is to just fold in the higher bits
with XOR.

This is independent of possible improvements to full_name_hash()
in future.

Signed-off-by: Stephen Hemminger <shemminger@...tta.com>


--- a/net/core/dev.c	2009-10-27 09:21:46.127252547 -0700
+++ b/net/core/dev.c	2009-10-27 09:25:14.593313378 -0700
@@ -199,7 +199,11 @@ EXPORT_SYMBOL(dev_base_lock);
 static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)
 {
 	unsigned hash = full_name_hash(name, strnlen(name, IFNAMSIZ));
-	return &net->dev_name_head[hash & ((1 << NETDEV_HASHBITS) - 1)];
+
+	hash ^= (hash >> NETDEV_HASHBITS);
+	hash &= NETDEV_HASHENTRIES - 1;
+
+	return &net->dev_name_head[hash];
 }
 
 static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)
--
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