netdev - Re: Why does tcp collapse behavior depend on nr

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Message-ID: <1478007189.7065.331.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 01 Nov 2016 06:33:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilya Lesokhin <ilyal@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        tls-fpga-sw-dev <tls-fpga-sw-dev@...lanox.com>,
        Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: Why does tcp collapse behavior depend on nr_frags?

On Tue, 2016-11-01 at 08:15 +0000, Ilya Lesokhin wrote:
> Hi,
> I've notice that tcp_can_collapse() returns false if skb_shinfo(skb)->nr_frags != 0.
> Is there a reason why we want to base the collapse decision in retransmission on whether
> the data is located in a frag or the linear part?
> 
> The relevant commit is
> 	 tcp: collapse more than two on retransmission  ('4a17fc3add594fcc1c778e93a95b6ecf47f630e5')
> 
> Thanks,
> Ilya

Good point.

I am guessing you see this after commit
3613b3dbd1ade9a6a626dae1f608c57638eb5e8a
("tcp: prepare skbs for better sack shifting")  ?

Problem is that the left skb might have no skb_availroom() anyway...

We could theoretically use the same helpers we use at sack shifting,
but are these collapse events frequent enough we should care ?

Thanks !