netdev - Re: [PATCH v2 0/2] Tracepoint for tcp retransmission

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Message-ID: <20120206152019.GG27935@redhat.com>
Date:	Mon, 6 Feb 2012 10:20:19 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>,
	"Frank Ch. Eigler" <fche@...stic.org>,
	Satoru Moriya <satoru.moriya@....com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"tgraf@...radead.org" <tgraf@...radead.org>,
	"stephen.hemminger@...tta.com" <stephen.hemminger@...tta.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	Seiji Aguchi <seiji.aguchi@....com>, fche@...rceware.org,
	mathieu.desnoyers@...ymtl.ca, rostedt@...dmis.org
Subject: Re: [PATCH v2 0/2] Tracepoint for tcp retransmission

Hi, Neil -

On Sun, Feb 05, 2012 at 08:32:47PM -0500, Neil Horman wrote:

> [...]
> > Hey Neil, I though about this more deeply. In my first iteration I
> > had the same answer. But IMHO if you think about this more deeply,
> > I think this is not a scalable approach (implementing in userspace
> > as systemtap scripts compared to implement them in kernelspace
> > (tracepoints). Why? Because at a deep TCP level the API (and thus
> > the context) is not static and the meaning of return values (or
> > arguments) change over time. No matter _where_ it is implemented
> > or coded.

> Yes, this is why I'm suggesting alternatives to the tracepoint.

I believe Hagen's point was that alternatives to tracepoints would
have substantially the same difficulty.  At some point in the stack,
you have to standardize/abstract.  What you hook up *at that point* is
not too important: tracepoints, or function calls, or expanded perf
machinery all seem to have the same burden.


> [...] If something is needed in the short term for Satoru, I still
> think a privately maintained module that registers a netfilter hook
> to watch outgoing packets for retransmits offers a good solution.

Does this mean that this netfilter hook mechanism is sufficient to
adapt to the current/future diversity of behaviors?  Why not make
*that* into a tracepoint then, so perf/stap scripts could get at it in
userspace?


- FChE
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