tcp: add TCP_RFC7323_PAWS_ACK drop reason

XPS can cause reorders because of the relaxed OOO
conditions for pure ACK packets.

For hosts not using RFS, what can happpen is that ACK
packets are sent on behalf of the cpu processing NIC
interrupts, selecting TX queue A for ACK packet P1.

Then a subsequent sendmsg() can run on another cpu.
TX queue selection uses the socket hash and can choose
another queue B for packets P2 (with payload).

If queue A is more congested than queue B,
the ACK packet P1 could be sent on the wire after
P2.

A linux receiver when processing P1 (after P2) currently increments
LINUX_MIB_PAWSESTABREJECTED (TcpExtPAWSEstab)
and use TCP_RFC7323_PAWS drop reason.
It might also send a DUPACK if not rate limited.

In order to better understand this pattern, this
patch adds a new drop_reason : TCP_RFC7323_PAWS_ACK.

For old ACKS like these, we no longer increment
LINUX_MIB_PAWSESTABREJECTED and no longer sends a DUPACK,
keeping credit for other more interesting DUPACK.

perf record -e skb:kfree_skb -a
perf script
...
         swapper       0 [148] 27475.438637: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
         swapper       0 [208] 27475.438706: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
         swapper       0 [208] 27475.438908: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
         swapper       0 [148] 27475.439010: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
         swapper       0 [148] 27475.439214: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
         swapper       0 [208] 27475.439286: skb:kfree_skb: ... location=tcp_validate_incoming+0x4f0 reason: TCP_RFC7323_PAWS_ACK
...

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20250113135558.3180360-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/dropreason-core.h

@@ -36,6 +36,7 @@
 	FN(TCP_OVERWINDOW)		\
 	FN(TCP_OFOMERGE)		\
 	FN(TCP_RFC7323_PAWS)		\
+	FN(TCP_RFC7323_PAWS_ACK)	\
 	FN(TCP_OLD_SEQUENCE)		\
 	FN(TCP_INVALID_SEQUENCE)	\
 	FN(TCP_INVALID_ACK_SEQUENCE)	\
@@ -259,6 +260,10 @@ enum skb_drop_reason {
 	 * LINUX_MIB_PAWSESTABREJECTED, LINUX_MIB_PAWSACTIVEREJECTED
 	 */
 	SKB_DROP_REASON_TCP_RFC7323_PAWS,
+	/**
+	 * @SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK: PAWS check, old ACK packet.
+	 */
+	SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK,
 	/** @SKB_DROP_REASON_TCP_OLD_SEQUENCE: Old SEQ field (duplicate packet) */
 	SKB_DROP_REASON_TCP_OLD_SEQUENCE,
 	/** @SKB_DROP_REASON_TCP_INVALID_SEQUENCE: Not acceptable SEQ field */

net/ipv4/tcp_input.c

@@ -4465,7 +4465,9 @@ static enum skb_drop_reason tcp_disordered_ack_check(const struct sock *sk,
 
 	/* 2. Is its sequence not the expected one ? */
 	if (seq != tp->rcv_nxt)
-		return reason;
+		return before(seq, tp->rcv_nxt) ?
+			SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK :
+			reason;
 
 	/* 3. Is this not a duplicate ACK ? */
 	if (ack != tp->snd_una)
@@ -5967,6 +5969,12 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	if (unlikely(th->syn))
 		goto syn_challenge;
 
+	/* Old ACK are common, do not change PAWSESTABREJECTED
+	 * and do not send a dupack.
+	 */
+	if (reason == SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK)
+		goto discard;
+
 	NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 	if (!tcp_oow_rate_limited(sock_net(sk), skb,
 				  LINUX_MIB_TCPACKSKIPPEDPAWS,