netdev - Re: [net-next] tcp: do tcp_mstamp_refresh before retransmits on TSQ handler

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Date:   Sun, 22 Oct 2017 20:40:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Koichiro Den <den@...ipeden.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, ncardwell@...gle.com
Subject: Re: [net-next] tcp: do tcp_mstamp_refresh before retransmits on TSQ
 handler

On Mon, 2017-10-23 at 12:26 +0900, Koichiro Den wrote:

> Now I wonder this is more of a theoretical one rather than a patch to fix one
> specific bug.

Note that I said that your patch was fine and I added a 'Reviewed-by:'
tag.

What I meant is that it has no direct effect on correctness of TCP
stack. I could not cook a packetdrill test that shows the difference
before and after your patch.

BTW, in the following sequence :

A)   Fetch high-res timestamp and store in X
B)   Use X

B) Can use a quite old value of X, depending on scheduling (preempt
kernels or interrupt handling)

TCP really does not care of how accurate X is, it is a best effort.

For RTX packets, it is even more the case, since TCP does not take RTT
samples from packets that were retransmitted.