net: force dst_default_metrics to const section

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@google.com> Cc: Ben Hutchings <bhutchings@solarflare.com> Signed-off-by: David S. Miller <davem@davemloft.net>
git-mirror · Aug 8, 2012 · a37e6e3 · a37e6e3
1 parent 0c03eca
commit a37e6e3
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Showing 2 changed files with 10 additions and 2 deletions.
diff --git a/include/net/dst.h b/include/net/dst.h
@@ -110,7 +110,7 @@ struct dst_entry {
 };
 
 extern u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old);
-extern const u32 dst_default_metrics[RTAX_MAX];
+extern const u32 dst_default_metrics[];
 
 #define DST_METRICS_READ_ONLY	0x1UL
 #define __DST_METRICS_PTR(Y)	\

diff --git 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 + 1] = {
+	/* 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.
+	 */
+	[RTAX_MAX] = 0xdeadbeef,
+};
+
 
 void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
 		int initial_ref, int initial_obsolete, unsigned short flags)