fib: introduce fib_alias_accessed() helper

Perf tools session at NFWS 2010 pointed out a false sharing on struct
fib_alias that can be avoided pretty easily, if we set FA_S_ACCESSED bit
only if needed (ie : not already set)
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_hash.c

@@ -335,7 +335,8 @@ void fib_table_select_default(struct fib_table *tb,
 			if (!next_fi->fib_nh[0].nh_gw ||
 			    next_fi->fib_nh[0].nh_scope != RT_SCOPE_LINK)
 				continue;
-			fa->fa_state |= FA_S_ACCESSED;
+
+			fib_alias_accessed(fa);
 
 			if (fi == NULL) {
 				if (next_fi != res->fi)

net/ipv4/fib_lookup.h

@@ -17,6 +17,13 @@ struct fib_alias {
 
 #define FA_S_ACCESSED	0x01
 
+/* Dont write on fa_state unless needed, to keep it shared on all cpus */
+static inline void fib_alias_accessed(struct fib_alias *fa)
+{
+	if (!(fa->fa_state & FA_S_ACCESSED))
+		fa->fa_state |= FA_S_ACCESSED;
+}
+
 /* Exported by fib_semantics.c */
 extern int fib_semantic_match(struct list_head *head,
 			      const struct flowi *flp,

net/ipv4/fib_semantics.c

@@ -901,7 +901,7 @@ int fib_semantic_match(struct list_head *head, const struct flowi *flp,
 		if (fa->fa_scope < flp->fl4_scope)
 			continue;
 
-		fa->fa_state |= FA_S_ACCESSED;
+		fib_alias_accessed(fa);
 
 		err = fib_props[fa->fa_type].error;
 		if (err == 0) {

net/ipv4/fib_trie.c

@@ -1838,7 +1838,8 @@ void fib_table_select_default(struct fib_table *tb,
 		if (!next_fi->fib_nh[0].nh_gw ||
 		    next_fi->fib_nh[0].nh_scope != RT_SCOPE_LINK)
 			continue;
-		fa->fa_state |= FA_S_ACCESSED;
+
+		fib_alias_accessed(fa);
 
 		if (fi == NULL) {
 			if (next_fi != res->fi)