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>

diff --git a/include/net/dst.h b/include/net/dst.h
index baf59789006427bc7c373449b62b271e6e777d15..621e3513ef5ed2c833a16902761b3d5c1ed2ebaf 100644 (file)
--- 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
index 069d51d29414a3245f9b60b66c534f78ce3b1752..56d63612e1e4b9a1ebfc04ddfa1bcd50333e0118 100644 (file)
--- 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)