netdev - Re: [RFC PATCH net-next 0/5] tcp: TCP tracer

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Message-Id: <1418770249.3446250.203741129.5333330E@webmail.messagingengine.com>
Date:	Tue, 16 Dec 2014 23:50:49 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>, jbaron@...mai.com
Cc:	jbacik@...com, eric.dumazet@...il.com,
	alexei.starovoitov@...il.com, chavey@...gle.com, ycheng@...gle.com,
	kafai@...com, netdev@...r.kernel.org, rostedt@...dmis.org,
	brakmo@...com, Kernel-team@...com,
	Daniel Borkmann <dborkman@...hat.com>,
	Florian Westphal <fw@...len.de>
Subject: Re: [RFC PATCH net-next 0/5] tcp: TCP tracer

On Tue, Dec 16, 2014, at 23:45, David Miller wrote:
> From: Jason Baron <jbaron@...mai.com>
> Date: Tue, 16 Dec 2014 17:40:47 -0500
> 
> > We are interested in tcp tracing as well. Another requirement that
> > we have that I don't think I saw is the ability to start/stop
> > tracing on sockets (potentially multiple times) during the lifetime
> > of a connection. So for example, the ability to use setsockopt(), to
> > selectively start/stop tracing on a connection, so as not to incur
> > overhead for non-traced sockets.
> 
> This is so backwards.
> 
> You make the tracing cheap enough that this can never be an issue.
> 
> Your requirement can only exist if the implementation is broken
> by design.

An idea I had was to add a proxy tcp congestion control which could be
selectively chosen per destination as soon as Daniel's patchset hits
net-next or one could enable it globally by
sys/net/ipv4/tcp_congestion_control. Needed tracepoints could be
installed in the congestion_ops handlers and the additional storage
could live inside the private data of the congestion control handler.
Further callbacks could go to a chained congestion control handler. The
names could be extended like "t:cubic", t for tracing.

Do the congestion control callbacks provide enough insight to the
connection state?

Bye,
Hannes
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