[PATCH V3 net-next] TLP: Don't reschedule PTO when there's one outstanding TLP retransmission

[Mao Wenan <maowenan@huawei.com>]

If there is one TLP probe went out(TLP use the write_queue_tail
packet as TLP probe, we assume this first TLP probe named A), and
this TLP probe was not acked by receive side.

Then the transmit side sent the next two packetes out(named B,C),
but unfortunately these two packets are also not acked by receive side.

And then there is one data packet with ack_seq A arrive at transmit
side, in tcp_ack() will call tcp_schedule_loss_probe() to rearm PTO,
the handler tcp_send_loss_probe() is to check
if(tp->tlp_high_seq) then go to rearm_timer(because there is one
outstanding TLP named A), so the new TLP probe can't be sent out and
it needs to rearm the RTO timer(timeout is relative to the transmit
time of the write queue head).

After that, there is another data packet with ack_seq A is received,
if the tlp_time_stamp is greater than rto_time_stamp, it will reset
the TLP timeout, which is before previous RTO timeout, so PTO is
rearm and previous RTO is cleared. Because there is no
retransmission packet was sent or no TLP sack receive,
tp->tlp_high_seq can't be reset to zero and the next TLP probe also
can't be sent out, so there is no way(or very long time)
to retransmit the lost packet.

This fix is to check(tp->tlp_high_seq) in tcp_schedule_loss_probe()
when TLP PTO is after RTO, It is not needed to reschedule PTO when
there is one outstanding TLP retransmission, so if the TLP A is lost
RTO can retransmit lost packet, then tp->tlp_high_seq will be set to
0, and TLP will go to the normal work process.

v1->v2
	refine some words of code and patch comments.
v2->v3
	delete senseless "{" and "}" in if clause.

Signed-off-by: Mao Wenan <maowenan@huawei.com>
---
 net/ipv4/tcp_output.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 886d874..b59975f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2377,6 +2377,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 timeout, tlp_time_stamp, rto_time_stamp;
+	s32 delta;
 
 	/* No consecutive loss probes. */
 	if (WARN_ON(icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)) {
@@ -2423,7 +2424,12 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 	tlp_time_stamp = tcp_jiffies32 + timeout;
 	rto_time_stamp = (u32)inet_csk(sk)->icsk_timeout;
 	if ((s32)(tlp_time_stamp - rto_time_stamp) > 0) {
-		s32 delta = rto_time_stamp - tcp_jiffies32;
+		/* It is not needed to reschedule PTO when there 
+		 * is one outstanding TLP retransmission. 
+		 */
+		if (tp->tlp_high_seq)
+			return false;
+		delta = rto_time_stamp - tcp_jiffies32;
 		if (delta > 0)
 			timeout = delta;
 	}
-- 
2.5.0