tcp: tsq: relax tcp_small_queue_check() when rtx queue contains a single skb

In commit 75eefc6c ("tcp: tsq: add a shortcut in tcp_small_queue_check()")
we allowed to send an skb regardless of TSQ limits being hit if rtx queue
was empty or had a single skb, in order to better fill the pipe
when/if TX completions were slow.

Then later, commit 75c119af ("tcp: implement rb-tree based
retransmit queue") accidentally removed the special case for
one skb in rtx queue.

Stefan Wahren reported a regression in single TCP flow throughput
using a 100Mbit fec link, starting from commit 65466904 ("tcp: adjust
TSO packet sizes based on min_rtt"). This last commit only made the
regression more visible, because it locked the TCP flow on a particular
behavior where TSQ prevented two skbs being pushed downstream,
adding silences on the wire between each TSO packet.

Many thanks to Stefan for his invaluable help !

Fixes: 75c119af ("tcp: implement rb-tree based retransmit queue")
Link: https://lore.kernel.org/netdev/7f31ddc8-9971-495e-a1f6-819df542e0af@gmx.net/Reported-by: Stefan Wahren <wahrenst@gmx.net>
Tested-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20231017124526.4060202-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_output.c

@@ -2542,6 +2542,18 @@ static bool tcp_pacing_check(struct sock *sk)
 	return true;
 }
 
+static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
+{
+	const struct rb_node *node = sk->tcp_rtx_queue.rb_node;
+
+	/* No skb in the rtx queue. */
+	if (!node)
+		return true;
+
+	/* Only one skb in rtx queue. */
+	return !node->rb_left && !node->rb_right;
+}
+
 /* TCP Small Queues :
  * Control number of packets in qdisc/devices to two packets / or ~1 ms.
  * (These limits are doubled for retransmits)
@@ -2579,12 +2591,12 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
 		limit += extra_bytes;
 	}
 	if (refcount_read(&sk->sk_wmem_alloc) > limit) {
-		/* Always send skb if rtx queue is empty.
+		/* Always send skb if rtx queue is empty or has one skb.
 		 * No need to wait for TX completion to call us back,
 		 * after softirq/tasklet schedule.
 		 * This helps when TX completions are delayed too much.
 		 */
-		if (tcp_rtx_queue_empty(sk))
+		if (tcp_rtx_queue_empty_or_single_skb(sk))
 			return false;
 
 		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);