tcp: Change txhash on every SYN and RTO retransmit

The current code changes txhash (flowlables) on every retransmitted
SYN/ACK, but only after the 2nd retransmitted SYN and only after
tcp_retries1 RTO retransmits.

With this patch:
1) txhash is changed with every SYN retransmits
2) txhash is changed with every RTO.

The result is that we can start re-routing around failed (or very
congested paths) as soon as possible. Otherwise application health
checks may fail and the connection may be terminated before we start
to change txhash.

v4: Removed sysctl, txhash is changed for all RTOs
v3: Removed text saying default value of sysctl is 0 (it is 100)
v2: Added sysctl documentation and cleaned code

Tested with packetdrill tests
Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -192,6 +192,8 @@ static int tcp_write_timeout(struct sock *sk)
 			if (tp->syn_data && icsk->icsk_retransmits == 1)
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPFASTOPENACTIVEFAIL);
+		} else if (!tp->syn_data && !tp->syn_fastopen) {
+			sk_rethink_txhash(sk);
 		}
 		retry_until = icsk->icsk_syn_retries ? : net->ipv4.sysctl_tcp_syn_retries;
 		syn_set = true;
@@ -213,6 +215,8 @@ static int tcp_write_timeout(struct sock *sk)
 			tcp_mtu_probing(icsk, sk);
 
 			dst_negative_advice(sk);
+		} else {
+			sk_rethink_txhash(sk);
 		}
 
 		retry_until = net->ipv4.sysctl_tcp_retries2;