netdev - [PATCH net-next v3 2/2] tcp: ack immediately when a cwr packet arrives

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Date:   Tue, 3 Jul 2018 09:26:15 -0700
From:   Lawrence Brakmo <brakmo@...com>
To:     netdev <netdev@...r.kernel.org>
CC:     Kernel Team <kernel-team@...com>, Blake Matheny <bmatheny@...com>,
        Alexei Starovoitov <ast@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Steve Ibanez <sibanez@...nford.edu>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next v3 2/2] tcp: ack immediately when a cwr packet arrives

We observed high 99 and 99.9% latencies when doing RPCs with DCTCP. The
problem is triggered when the last packet of a request arrives CE
marked. The reply will carry the ECE mark causing TCP to shrink its cwnd
to 1 (because there are no packets in flight). When the 1st packet of
the next request arrives, the ACK was sometimes delayed even though it
is CWR marked, adding up to 40ms to the RPC latency.

This patch insures that CWR makred data packets arriving will be acked
immediately.

Modified based on comments by Neal Cardwell <ncardwell@...gle.com>

Signed-off-by: Lawrence Brakmo <brakmo@...com>
---
 net/ipv4/tcp_input.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2c5d70bc294e..23c2a43de8a4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -246,8 +246,15 @@ static void tcp_ecn_queue_cwr(struct tcp_sock *tp)
 
 static void tcp_ecn_accept_cwr(struct tcp_sock *tp, const struct sk_buff *skb)
 {
-	if (tcp_hdr(skb)->cwr)
+	if (tcp_hdr(skb)->cwr) {
 		tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
+
+		/* If the sender is telling us it has entered CWR, then its
+		 * cwnd may be very low (even just 1 packet), so we should ACK
+		 * immediately.
+		 */
+		tcp_enter_quickack_mode((struct sock *)tp, 2);
+	}
 }
 
 static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp)
-- 
2.17.1