netdev - Re: [PATCH net] tcp: fix child sockets to use system default congestion control if not set

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Date:	Fri, 29 May 2015 20:24:25 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, Florian Westphal <fw@...len.de>,
	Glenn Judd <glenn.judd@...ganstanley.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net] tcp: fix child sockets to use system default congestion
 control if not set

On 05/29/2015 07:47 PM, Neal Cardwell wrote:
> Linux 3.17 and earlier are explicitly engineered so that if the app
> doesn't specifically request a CC module on a listener before the SYN
> arrives, then the child gets the system default CC when the connection
> is established. See tcp_init_congestion_control() in 3.17 or earlier,
> which says "if no choice made yet assign the current value set as
> default". The change ("net: tcp: assign tcp cong_ops when tcp sk is
> created") altered these semantics, so that children got their parent
> listener's congestion control even if the system default had changed
> after the listener was created.
>
> This commit returns to those original semantics from 3.17 and earlier,
> since they are the original semantics from 2007 in 4d4d3d1e8 ("[TCP]:
> Congestion control initialization."), and some Linux congestion
> control workflows depend on that.
>
> In summary, if a listener socket specifically sets TCP_CONGESTION to
> "x", or the route locks the CC module to "x", then the child gets
> "x". Otherwise the child gets current system default from
> net.ipv4.tcp_congestion_control. That's the behavior in 3.17 and
> earlier, and this commit restores that.
>
> Fixes: 55d8694fa82c ("net: tcp: assign tcp cong_ops when tcp sk is created")
> Cc: Florian Westphal <fw@...len.de>
> Cc: Daniel Borkmann <dborkman@...hat.com>
> Cc: Glenn Judd <glenn.judd@...ganstanley.com>
> Cc: Stephen Hemminger <stephen@...workplumber.org>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>

Ok, change looks good to me, thanks.

Acked-by: Daniel Borkmann <daniel@...earbox.net>
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