Date:	Thu, 18 Dec 2014 17:42:20 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Lawrence Brakmo <brakmo@...com>
Cc:	Josef Bacik <jbacik@...com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Martin Lau <kafai@...com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Blake Matheny <bmatheny@...com>,
	Laurent Chavey <chavey@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Kernel Team <Kernel-team@...com>
Subject: Re: [RFC PATCH net-next 0/5] tcp: TCP tracer

On Thu, Dec 18, 2014 at 3:43 PM, Lawrence Brakmo <brakmo@...com> wrote:
>
>
> On 12/17/14, 1:42 PM, "Josef Bacik" <jbacik@...com> wrote:
>
> >>>> It feels that for stats collection only, tracepoints+tcp_trace
> >>>> do not add much additional value vs extending tcp_info
> >>>> and using ss.
> >>> I think we are on the same page. Once 'this should cost nothing if not
> >>> activated' proposition was cleared out.  It was what I meant that
> >>>doing the
> >>> collection part in the TCP itself (instead of tracepoints) would be
> >>>nice.
> >>
> >> agree.
> >>
> >>> I think going forward, as others have suggested, it may be better to
> >>>come
> >>> together and reach a common ground on what to collect first before I
> >>>re-work
> >>> patch 1 to 3 and repost.
> >>
> >> I think as a minimum it will be discussed at netdev01 in Feb,
> >> but I suspect not everyone on this list can(want) go to Ottawa,
> >> so would be nice to have a meetup for bay area folks to
> >> discuss this sooner with public g+ hangout.
> >> Thoughts?
> >>
> >
> >Yeah I think we're all in agreement that this is a good netdev01
> >discussion.  I'm happy to include people who want to talk about this
> >before hand in the bay area meetup we're throwing, but it seems like
> >this is going to be something that the larger community is going to want
> >to talk about so it may be more productive to wait until netdev01.
> >Thanks,
>
>
> Josef: I think a preliminary discussion during the bay area meet up would
> be useful to get some of us in sync.
>
> There are two issues going on. One is the collection of statistics that
> can be read every-so-often and another is the issue of enabling easier
> tracing of TCP state for analysis and debugging.
>
> For statistics collection, extending tcp_info is a viable option although
> we may need to do some modifications to deal with: (1) Having many
> connections most of which are idle. We need an option to only output those
> whose stats have changed since the last read. (2) A mechanism to deal with
> closed connections and their stats. Note that in our current setup neither
> of these is an issue for us.
>
> For tracing and event collection, I see a lot of value in tracepoints that
> could print basic info with perf but also allow us to do more complex
> things by loading a module that hooks to the tracepoints. This is one way
> to set up triggers to collect state for a particular flow.
>
> Yuchung: I agree that a lot of information can be obtained through
> analysis of tcpdumps, but some internal state must be inferred and in many
> instances we can only get bounds.
Hi Larry :)

I definitely see values in tracepoints. I was responding to the commit
message in patch 5/5: "Uncover uplink/backbone/subnet issue, e.g. by
tracking the rxmit rate.". First ss and tcp_info can collect that
data. But rxmit rate is often not enough for diagnosis. One needs to
inspect the loss patterns from packet traces. I am sure you know what
I am talking about.



>
> - Larry
>
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