[PATCH net] tcp: fix TLP timer not set when CA_STATE changes from DISORDER to OPEN

[Pengcheng Yang <yangpc@wangsu.com>]

When CA_STATE is in DISORDER, the TLP timer is not set when receiving
an ACK (a cumulative ACK covered out-of-order data) causes CA_STATE to
change from DISORDER to OPEN. If the sender is app-limited, it can only
wait for the RTO timer to expire and retransmit.

The reason for this is that the TLP timer is set before CA_STATE changes
in tcp_ack(), so we delay the time point of calling tcp_set_xmit_timer()
until after tcp_fastretrans_alert() returns and remove the
FLAG_SET_XMIT_TIMER from ack_flag when the RACK reorder timer is set.

This commit has two additional benefits:
1) Make sure to reset RTO according to RFC6298 when receiving ACK, to
avoid spurious RTO caused by RTO timer early expires.
2) Reduce the xmit timer reschedule once per ACK when the RACK reorder
timer is set.

Link: https://lore.kernel.org/netdev/1611139794-11254-1-git-send-email-yangpc@wangsu.com
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Cc: Neal Cardwell <ncardwell@google.com>
---
 include/net/tcp.h       |  2 +-
 net/ipv4/tcp_input.c    | 10 ++++++----
 net/ipv4/tcp_recovery.c |  5 +++--
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 78d13c8..67f7e52 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -2060,7 +2060,7 @@ static inline __u32 cookie_init_sequence(const struct tcp_request_sock_ops *ops,
 void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced);
 extern s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb,
 				u32 reo_wnd);
-extern void tcp_rack_mark_lost(struct sock *sk);
+extern bool tcp_rack_mark_lost(struct sock *sk);
 extern void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 			     u64 xmit_time);
 extern void tcp_rack_reo_timeout(struct sock *sk);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c7e16b0..d0a9588 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2859,7 +2859,8 @@ static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag)
 	} else if (tcp_is_rack(sk)) {
 		u32 prior_retrans = tp->retrans_out;
 
-		tcp_rack_mark_lost(sk);
+		if (tcp_rack_mark_lost(sk))
+			*ack_flag &= ~FLAG_SET_XMIT_TIMER;
 		if (prior_retrans > tp->retrans_out)
 			*ack_flag |= FLAG_LOST_RETRANS;
 	}
@@ -3815,9 +3816,6 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 
 	if (tp->tlp_high_seq)
 		tcp_process_tlp_ack(sk, ack, flag);
-	/* If needed, reset TLP/RTO timer; RACK may later override this. */
-	if (flag & FLAG_SET_XMIT_TIMER)
-		tcp_set_xmit_timer(sk);
 
 	if (tcp_ack_is_dubious(sk, flag)) {
 		if (!(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP))) {
@@ -3830,6 +3828,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 				      &rexmit);
 	}
 
+	/* If needed, reset TLP/RTO timer when RACK doesn't set. */
+	if (flag & FLAG_SET_XMIT_TIMER)
+		tcp_set_xmit_timer(sk);
+
 	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
 		sk_dst_confirm(sk);
 
diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c
index 177307a..6f1b4ac 100644
--- a/net/ipv4/tcp_recovery.c
+++ b/net/ipv4/tcp_recovery.c
@@ -96,13 +96,13 @@ static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
 	}
 }
 
-void tcp_rack_mark_lost(struct sock *sk)
+bool tcp_rack_mark_lost(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 timeout;
 
 	if (!tp->rack.advanced)
-		return;
+		return false;
 
 	/* Reset the advanced flag to avoid unnecessary queue scanning */
 	tp->rack.advanced = 0;
@@ -112,6 +112,7 @@ void tcp_rack_mark_lost(struct sock *sk)
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
 					  timeout, inet_csk(sk)->icsk_rto);
 	}
+	return !!timeout;
 }
 
 /* Record the most recently (re)sent time among the (s)acked packets
-- 
1.8.3.1