[PATCH net-next] tcp: use likely() for srtt check in tcp_rtt_estimator()

In tcp_rtt_estimator(), srtt is initialized to a non-zero value
after the first RTT measurement (srtt = m << 3) and never returns
to zero for the lifetime of the connection. Thus, the "srtt != 0"
branch is almost always true.

Marking it with likely() helps the compiler generate better code
for this hot path.

No functional change is introduced.

Signed-off-by: Ziran Zhang <[email protected]>
---
 net/ipv4/tcp_input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5b6378b94..a4c4d4760 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1089,7 +1089,7 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 	 * does not matter how to _calculate_ it. Seems, it was trap
 	 * that VJ failed to avoid. 8)
 	 */
-	if (srtt != 0) {
+	if (likely(srtt != 0)) {
 		m -= (srtt >> 3);	/* m is now error in rtt est */
 		srtt += m;		/* rtt = 7/8 rtt + 1/8 new */
 		if (m < 0) {
-- 
2.51.0