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: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 12 insertions, 3 deletions

diff --git a/net/ipv4/fib_hash.c b/net/ipv4/fib_hash.c
index 4f1aafd3ba89..43e1c594ce8f 100644
--- a/net/ipv4/fib_hash.c
+++ b/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)
diff --git a/net/ipv4/fib_lookup.h b/net/ipv4/fib_lookup.h
index 5072d8effd5d..a29edf2219c8 100644
--- a/net/ipv4/fib_lookup.h
+++ b/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,
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 6734c9cab248..3e0da3ef6116 100644
--- a/net/ipv4/fib_semantics.c
+++ b/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) {
diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 31494f335686..cd5e13aee7d5 100644
--- a/net/ipv4/fib_trie.c
+++ b/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)