netdev - [PATCH] net: force dst_default

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Date:	Tue, 07 Aug 2012 18:11:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: [PATCH] net: force dst_default_metrics to const section

From: Eric Dumazet <edumazet@...gle.com>

While investigating on network performance problems, I found this little
gem :

$ nm -v vmlinux | grep -1 dst_default_metrics
ffffffff82736540 b busy.46605 
ffffffff82736560 B dst_default_metrics
ffffffff82736598 b dst_busy_list

Apparently, declaring a const array without initializer put it in
(writeable) bss section, in middle of possibly often dirtied cache
lines.

Since we really want dst_default_metrics be const to avoid any possible
false sharing and catch any buggy writes, I force a null initializer.

ffffffff818a4c20 R dst_default_metrics

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
For reference, dst_default_metrics was added in 2.6.39

 net/core/dst.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/core/dst.c b/net/core/dst.c
index 069d51d..4c538be 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -149,7 +149,15 @@ int dst_discard(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(dst_discard);
 
-const u32 dst_default_metrics[RTAX_MAX];
+const u32 dst_default_metrics[RTAX_MAX] = {
+	/* This initializer is needed to force linker to place this variable
+	 * into const section. Otherwise it might end into bss section.
+	 * We really want to avoid false sharing on this variable, and catch
+	 * any writes on it.
+	 */
+	0
+};
+
 
 void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
 		int initial_ref, int initial_obsolete, unsigned short flags)


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