netdev - [PATCH net-next 2/3] tcp: tcp_sack_new_ofo

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Date:   Thu, 30 Apr 2020 10:35:42 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 2/3] tcp: tcp_sack_new_ofo_skb() should be more conservative

Currently, tcp_sack_new_ofo_skb() sends an ack if prior
acks were 'compressed', if room has to be made in tp->selective_acks[]

But there is no guarantee all four sack ranges can be included
in SACK option. As a matter of fact, when TCP timestamps option
is used, only three SACK ranges can be included.

Lets assume only two ranges can be included, and force the ack:

- When we touch more than 2 ranges in the reordering
  done if tcp_sack_extend() could be done.

- If we have at least 2 ranges when adding a new one.

This enforces that before a range is in third or fourth
position, at least one ACK packet included it in first/second
position.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Neal Cardwell <ncardwell@...gle.com>
---
 net/ipv4/tcp_input.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index da777df0a0baefb3bef8c802c9b9b83ff38b9fc9..ef921ecba4155abe9d078152ca8bd6a0be68317e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4348,6 +4348,12 @@ static void tcp_sack_compress_send_ack(struct sock *sk)
 	tcp_send_ack(sk);
 }
 
+/* Reasonable amount of sack blocks included in TCP SACK option
+ * The max is 4, but this becomes 3 if TCP timestamps are there.
+ * Given that SACK packets might be lost, be conservative and use 2.
+ */
+#define TCP_SACK_BLOCKS_EXPECTED 2
+
 static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -4360,6 +4366,8 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 
 	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
 		if (tcp_sack_extend(sp, seq, end_seq)) {
+			if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+				tcp_sack_compress_send_ack(sk);
 			/* Rotate this_sack to the first one. */
 			for (; this_sack > 0; this_sack--, sp--)
 				swap(*sp, *(sp - 1));
@@ -4369,6 +4377,9 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 		}
 	}
 
+	if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+		tcp_sack_compress_send_ack(sk);
+
 	/* Could not find an adjacent existing SACK, build a new one,
 	 * put it at the front, and shift everyone else down.  We
 	 * always know there is at least one SACK present already here.
@@ -4376,7 +4387,6 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 	 * If the sack array is full, forget about the last one.
 	 */
 	if (this_sack >= TCP_NUM_SACKS) {
-		tcp_sack_compress_send_ack(sk);
 		this_sack--;
 		tp->rx_opt.num_sacks--;
 		sp--;
-- 
2.26.2.303.gf8c07b1a785-goog