linux-kernel - Re: [PATCH net-next] tcp: add retransmission quit reasons to tracepoint

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Message-ID: <CANn89iK-6kT-ZUpNRMjPY9_TkQj-dLuKrDQtvO1140q4EUsjFg@mail.gmail.com>
Date: Mon, 30 Jun 2025 02:11:42 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: xu.xin16@....com.cn
Cc: kuba@...nel.org, kuniyu@...zon.com, ncardwell@...gle.com, 
	davem@...emloft.net, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, yang.yang29@....com.cn, 
	fan.yu9@....com.cn, tu.qiang35@....com.cn, jiang.kun2@....com.cn
Subject: Re: [PATCH net-next] tcp: add retransmission quit reasons to tracepoint

On Mon, Jun 30, 2025 at 1:05 AM <xu.xin16@....com.cn> wrote:
>
> From: Fan Yu <fan.yu9@....com.cn>
>
> Problem
> =======
> When TCP retransmits a packet due to missing ACKs, the retransmission
> may fail for various reasons (e.g., packets stuck in driver queues,
> sequence errors, or routing issues). Currently, these failure reasons
> are internally handled in __tcp_retransmit_skb() but lack visibility to
> userspace, which makes it difficult to diagnose retransmission failures in
> production environments.
>
> Solution
> =======
> This patch adds a reason field to the tcp_retransmit_skb tracepoint,
> enumerating with explicit failure cases:
> TCP_RETRANS_IN_HOST_QUEUE          (packet still queued in driver)
> TCP_RETRANS_END_SEQ_ERROR          (invalid end sequence)
> TCP_RETRANS_TRIM_HEAD_NOMEM      (trim head no memory)
> TCP_RETRANS_UNCLONE_NOMEM    (skb unclone keeptruesize no memory)
> TCP_RETRANS_FRAG_NOMEM       (fragment no memory)
> TCP_RETRANS_ROUTE_FAIL       (routing failure)
> TCP_RETRANS_RCV_ZERO_WINDOW  (closed recevier window)
> TCP_RETRANS_PSKB_COPY_NOBUFS (no buffer for skb copy)
> TCP_RETRANS_QUIT_UNDEFINED   (quit reason undefined)

'undefined' ?

>
> Impact
> ======
> 1. Enables BPF programs to filter retransmission failures by reason.
> 2. Allows precise failure rate monitoring via ftrace.
>
> Co-developed-by: xu xin <xu.xin16@....com.cn>
> Signed-off-by: xu xin <xu.xin16@....com.cn>
> Signed-off-by: Fan Yu <fan.yu9@....com.cn>

Problem is that this patch breaks the original trace point, without
any mention of the potential consequences in the changelog ?

commit e086101b150ae8e99e54ab26101ef3835fa9f48d
Author: Cong Wang <xiyou.wangcong@...il.com>
Date:   Fri Oct 13 13:03:16 2017 -0700

    tcp: add a tracepoint for tcp retransmission