netdev - [PATCH net-next v2 1/2] tcp: notify when a delayed ack is sent

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Message-ID: <20180702213908.1246455-2-brakmo@fb.com>
Date:   Mon, 2 Jul 2018 14:39:07 -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 v2 1/2] tcp: notify when a delayed ack is sent

DCTCP depends on the CA_EVENT_NON_DELAYED_ACK and CA_EVENT_DELAYED_ACK
notifications to keep track if it needs to send an ACK for packets that
were received with a particular ECN state but whose ACK was delayed.

Under some circumstances, for example when a delayed ACK is sent with a
data packet, DCTCP state was not being updated due to a lack of
notification that the previously delayed ACK was sent. As a result, it
would sometimes send a duplicate ACK when a new data packet arrived.

This patch insures that DCTCP's state is correctly updated so it will
not send the duplicate ACK.

Improved based on comments from Neal Cardwell <ncardwell@...gle.com>.

Signed-off-by: Lawrence Brakmo <brakmo@...com>
---
 net/ipv4/tcp_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f8f6129160dd..acefb64e8280 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -172,6 +172,8 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
 			__sock_put(sk);
 	}
 	tcp_dec_quickack_mode(sk, pkts);
+	if (inet_csk_ack_scheduled(sk))
+		tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
 	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
 }

@@ -3567,8 +3569,6 @@ void tcp_send_ack(struct sock *sk)
 	if (sk->sk_state == TCP_CLOSE)
 		return;

-	tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
-
 	/* We are not putting this on the write queue, so
 	 * tcp_transmit_skb() will set the ownership to this
 	 * sock.
-- 
2.17.1