tcp: make cwnd-limited checks measurement-based, and gentler

Experience with the recent e114a710aa50 ("tcp: fix cwnd limited
checking to improve congestion control") has shown that there are
common cases where that commit can cause cwnd to be much larger than
necessary. This leads to TSO autosizing cooking skbs that are too
large, among other things.

The main problems seemed to be:

(1) That commit attempted to predict the future behavior of the
connection by looking at the write queue (if TSO or TSQ limit
sending). That prediction sometimes overestimated future outstanding
packets.

(2) That commit always allowed cwnd to grow to twice the number of
outstanding packets (even in congestion avoidance, where this is not
needed).

This commit improves both of these, by:

(1) Switching to a measurement-based approach where we explicitly
track the largest number of packets in flight during the past window
("max_packets_out"), and remember whether we were cwnd-limited at the
moment we finished sending that flight.

(2) Only allowing cwnd to grow to twice the number of outstanding
packets ("max_packets_out") in slow start. In congestion avoidance
mode we now only allow cwnd to grow if it was fully utilized.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 3 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index f5d6ca4a9d2..e80abe4486c 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -971,8 +971,9 @@ static inline u32 tcp_wnd_end(const struct tcp_sock *tp)
 
 /* We follow the spirit of RFC2861 to validate cwnd but implement a more
  * flexible approach. The RFC suggests cwnd should not be raised unless
- * it was fully used previously. But we allow cwnd to grow as long as the
- * application has used half the cwnd.
+ * it was fully used previously. And that's exactly what we do in
+ * congestion avoidance mode. But in slow start we allow cwnd to grow
+ * as long as the application has used half the cwnd.
  * Example :
  *    cwnd is 10 (IW10), but application sends 9 frames.
  *    We allow cwnd to reach 18 when all frames are ACKed.
@@ -985,7 +986,11 @@ static inline bool tcp_is_cwnd_limited(const struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 
-	return tp->snd_cwnd < 2 * tp->lsnd_pending;
+	/* If in slow start, ensure cwnd grows to twice what was ACKed. */
+	if (tp->snd_cwnd <= tp->snd_ssthresh)
+		return tp->snd_cwnd < 2 * tp->max_packets_out;
+
+	return tp->is_cwnd_limited;
 }
 
 static inline void tcp_check_probe_timer(struct sock *sk)