[PATCH 3.10 009/139] tcp: enable xmit timer fix by having TLP use time when RTO should fire

[Willy Tarreau]

From: Neal Cardwell <ncardwell@xxxxxxxxxx>

commit a2815817ffa68c7933a43eb55836d6e789bd4389 upstream.

Have tcp_schedule_loss_probe() base the TLP scheduling decision based
on when the RTO *should* fire. This is to enable the upcoming xmit
timer fix in this series, where tcp_schedule_loss_probe() cannot
assume that the last timer installed was an RTO timer (because we are
no longer doing the "rearm RTO, rearm RTO, rearm TLP" dance on every
ACK). So tcp_schedule_loss_probe() must independently figure out when
an RTO would want to fire.

In the new TLP implementation following in this series, we cannot
assume that icsk_timeout was set based on an RTO; after processing a
cumulative ACK the icsk_timeout we see can be from a previous TLP or
RTO. So we need to independently recalculate the RTO time (instead of
reading it out of icsk_timeout). Removing this dependency on the
nature of icsk_timeout makes things a little easier to reason about
anyway.

Note that the old and new code should be equivalent, since they are
both saying: "if the RTO is in the future, but at an earlier time than
the normal TLP time, then set the TLP timer to fire when the RTO would
have fired".

[This version of the commit was compiled and briefly tested
based on top of v3.10.107.]

Change-Id: I597ad6446edde15bf2cea8e56d603a2c52f8221b
Fixes: 6ba8a3b19e76 ("tcp: Tail loss probe (TLP)")
Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Nandita Dukkipati <nanditad@xxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
---
 net/ipv4/tcp_output.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8729a93..1354402 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1947,8 +1947,8 @@ 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;
 	u32 rtt = tp->srtt >> 3;
+	u32 timeout, rto_delta;
 
 	if (WARN_ON(icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS))
 		return false;
@@ -1987,14 +1987,10 @@ bool tcp_schedule_loss_probe(struct sock *sk)
 				(rtt + (rtt >> 1) + TCP_DELACK_MAX));
 	timeout = max_t(u32, timeout, msecs_to_jiffies(10));
 
-	/* If RTO is shorter, just schedule TLP in its place. */
-	tlp_time_stamp = tcp_time_stamp + 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_time_stamp;
-		if (delta > 0)
-			timeout = delta;
-	}
+	/* If the RTO formula yields an earlier time, then use that time. */
+	rto_delta = tcp_rto_delta(sk);  /* How far in future is RTO? */
+	if (rto_delta > 0)
+		timeout = min_t(u32, timeout, rto_delta);
 
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout,
 				  TCP_RTO_MAX);
-- 
2.8.0.rc2.1.gbe9624a