tcp: fix tcp_tso_should_defer() vs large RTT

Neal reported that using neper tcp_stream with TCP_TX_DELAY
set to 50ms would often lead to flows stuck in a small cwnd mode,
regardless of the congestion control.

While tcp_stream sets TCP_TX_DELAY too late after the connect(),
it highlighted two kernel bugs.

The following heuristic in tcp_tso_should_defer() seems wrong
for large RTT:

delta = tp->tcp_clock_cache - head->tstamp;
/* If next ACK is likely to come too late (half srtt), do not defer */
if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
goto send_now;

If next ACK is expected to come in more than 1 ms, we should
not defer because we prefer a smooth ACK clocking.

While blamed commit was a step in the good direction, it was not
generic enough.

Another patch fixing TCP_TX_DELAY for established flows
will be proposed when net-next reopens.

Fixes: 50c8339e9299 ("tcp: tso: restore IW10 after TSO autosizing")
Reported-by: Neal Cardwell <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Neal Cardwell <[e-mail]>
Tested-by: Neal Cardwell <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
[[e-mail]: fixed whitespace issue]
Signed-off-by: Paolo Abeni <[e-mail]>

net/ipv4/tcp_output.c

···
 				 u32 max_segs)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	u32 send_win, cong_win, limit, in_flight;
+	u32 send_win, cong_win, limit, in_flight, threshold;
+	u64 srtt_in_ns, expected_ack, how_far_is_the_ack;
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *head;
 	int win_divisor;
···
 	head = tcp_rtx_queue_head(sk);
 	if (!head)
 		goto send_now;
-	delta = tp->tcp_clock_cache - head->tstamp;
-	/* If next ACK is likely to come too late (half srtt), do not defer */
-	if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
+
+	srtt_in_ns = (u64)(NSEC_PER_USEC >> 3) * tp->srtt_us;
+	/* When is the ACK expected ? */
+	expected_ack = head->tstamp + srtt_in_ns;
+	/* How far from now is the ACK expected ? */
+	how_far_is_the_ack = expected_ack - tp->tcp_clock_cache;
+
+	/* If next ACK is likely to come too late,
+	 * ie in more than min(1ms, half srtt), do not defer.
+	 */
+	threshold = min(srtt_in_ns >> 1, NSEC_PER_MSEC);
+
+	if ((s64)(how_far_is_the_ack - threshold) > 0)
 		goto send_now;
 
 	/* Ok, it looks like it is advisable to defer.