netdev - Why does tcp collapse behavior depend on nr

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Message-ID: <VI1PR0502MB29577BA8C670D1379CC4F1B7D4A10@VI1PR0502MB2957.eurprd05.prod.outlook.com>
Date:   Tue, 1 Nov 2016 08:15:15 +0000
From:   Ilya Lesokhin <ilyal@...lanox.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        tls-fpga-sw-dev <tls-fpga-sw-dev@...lanox.com>
Subject: Why does tcp collapse behavior depend on nr_frags?

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