netdev - Re: [PATCH net-next] tcp: Add tracepoint for rxtstamp coalescing

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Date: Fri, 14 Jun 2024 10:09:51 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Philo Lu <lulie@...ux.alibaba.com>, netdev@...r.kernel.org, 
	edumazet@...gle.com
Cc: rostedt@...dmis.org, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com,  davem@...emloft.net, dsahern@...nel.org,
 kuba@...nel.org,  xuanzhuo@...ux.alibaba.com, dust.li@...ux.alibaba.com
Subject: Re: [PATCH net-next] tcp: Add tracepoint for rxtstamp coalescing

On Tue, 2024-06-11 at 12:58 +0800, Philo Lu wrote:
> During tcp coalescence, rx timestamps of the former skb ("to" in
> tcp_try_coalesce), will be lost. This may lead to inaccurate
> timestamping results if skbs come out of order.
> 
> Here is an example.
> Assume a message consists of 3 skbs, namely A, B, and C. And these skbs
> are processed by tcp in the following order:
> A -(1us)-> C -(1ms)-> B

IMHO the above order makes the changelog confusing

> If C is coalesced to B, the final rx timestamps of the message will be
> those of C. That is, the timestamps show that we received the message
> when C came (including hardware and software). However, we actually
> received it 1ms later (when B came).
> 
> With the added tracepoint, we can recognize such cases and report them
> if we want.

We really need very good reasons to add new tracepoints to TCP. I'm
unsure if the above example match such requirement. The reported
timestamp actually matches the first byte in the aggregate segment,
inferring anything more is IMHO stretching too far the API semantic.

Cheers,

Paolo