netdev - Re: [PATCH v2 net-next 2/3] tcp: add TCP_RFC7323_PAWS

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Message-ID: <CAL+tcoC4SqNZZ1tw3RHkMe1e2kdtwXK8_-=NFpiCAT9m3Ukm6Q@mail.gmail.com>
Date: Mon, 13 Jan 2025 23:02:35 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, Simon Horman <horms@...nel.org>, 
	Neal Cardwell <ncardwell@...gle.com>, Kuniyuki Iwashima <kuniyu@...zon.com>, eric.dumazet@...il.com
Subject: Re: [PATCH v2 net-next 2/3] tcp: add TCP_RFC7323_PAWS_ACK drop reason

On Mon, Jan 13, 2025 at 9:56 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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@...gle.com>
> Reviewed-by: Neal Cardwell <ncardwell@...gle.com>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Thank you, Eric.

Reviewed-by: Jason Xing <kerneljasonxing@...il.com>