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

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Message-ID: <20250110180301.57516-1-kuniyu@amazon.com>
Date: Sat, 11 Jan 2025 03:03:01 +0900
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <eric.dumazet@...il.com>, <horms@...nel.org>,
	<kuba@...nel.org>, <kuniyu@...zon.com>, <ncardwell@...gle.com>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH net-next 2/2] tcp: add TCP_RFC7323_PAWS_ACK drop reason

From: Eric Dumazet <edumazet@...gle.com>
Date: Fri, 10 Jan 2025 14:33:15 +0000
> 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 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: Kuniyuki Iwashima <kuniyu@...zon.com>