tcp: Add tcp_clamp_rto_to_user_timeout() helper to improve accuracy

[Jon Maxwell]

Create the tcp_clamp_rto_to_user_timeout() helper routine. To calculate
the correct rto, so that the TCP_USER_TIMEOUT socket option is more 
accurate. Taking suggestions and feedback into account from 
Eric Dumazet, Neal Cardwell and David Laight. Due to the 1st commit we 
can avoid the msecs_to_jiffies() and jiffies_to_msecs() dance.

Signed-off-by: Jon Maxwell <jmaxwell37@xxxxxxxxx>
---
 net/ipv4/tcp_timer.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index d212f183dd2d..a242f8874629 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -36,6 +36,21 @@ u32 tcp_retransmit_stamp(const struct sock *sk)
 	return start_ts;
 }
 
+static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	u32 elapsed, start_ts;
+
+	start_ts = tcp_retransmit_stamp(sk);
+	if (!icsk->icsk_user_timeout || !start_ts)
+		return icsk->icsk_rto;
+	elapsed = tcp_time_stamp(tcp_sk(sk)) - start_ts;
+	if (elapsed >= icsk->icsk_user_timeout)
+		return 1; /* user timeout has passed; fire ASAP */
+	else
+		return min_t(u32, icsk->icsk_rto, msecs_to_jiffies(icsk->icsk_user_timeout - elapsed));
+}
+
 /**
  *  tcp_write_err() - close socket and save error info
  *  @sk:  The socket the error has appeared on.
@@ -544,7 +559,8 @@ void tcp_retransmit_timer(struct sock *sk)
 		/* Use normal (exponential) backoff */
 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 	}
-	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
+	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+				  tcp_clamp_rto_to_user_timeout(sk), TCP_RTO_MAX);
 	if (retransmits_timed_out(sk, net->ipv4.sysctl_tcp_retries1 + 1, 0))
 		__sk_dst_reset(sk);
 
-- 
2.13.6