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>

1 files changed, 7 insertions, 0 deletions

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,