netdev - Re: [net-next 3/3] tcp: keep tcp_collapse controllable even after processing starts

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Date:   Sat, 14 Oct 2017 08:46:10 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Koichiro Den <den@...ipeden.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org
Subject: Re: [net-next 3/3] tcp: keep tcp_collapse controllable even after
 processing starts

On Sat, 2017-10-14 at 16:27 +0900, Koichiro Den wrote:
> Combining actual collapsing with reasoning for deciding the starting
> point, we can apply its logic in a consistent manner such that we can
> avoid costly yet not much useful collapsing. When collapsing to be
> triggered, it's not rare that most of the skbs in the receive or ooo
> queue are large ones without much metadata overhead. This also
> simplifies code and makes it easier to apply logic in a fair manner.
> 
> Subtle subsidiary changes included:
> - When the end_seq of the skb we are trying to collapse was larger than
>   the 'end' argument provided, we would end up copying to the 'end'
>   even though we couldn't collapse the original one. Current users of
>   tcp_collapse does not require such reserves so redefines it as the
>   point over which skbs whose seq passes guranteed not to be collapsed.
> - Naturally tcp_collapse_ofo_queue shapes up and we no longer need
>   'tail' argument.


I am not inclined to review such a large change, without you providing
actual numbers.

We have a problem in TCP right now, that receiver announces a too big
window, and that is the main reason we trigger collapsing.

I would rather fix the root cause.