linux-kernel - Re: [RFC][PATCH 1/5] [PATCH 1/5] events: Add EVENT

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Date:	Wed, 17 Nov 2010 10:16:59 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@....de>
Subject: Re: [RFC][PATCH 1/5] [PATCH 1/5] events: Add EVENT_FS the event
 filesystem

On Wed, 2010-11-17 at 16:03 +0100, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > Are these events now going to be labeled as stable?  Is every tracepoint we have, 
> > much have the same data?  Linus specifically said at Kernel Summit that he wants 
> > absolutely NO modules to have a stable tracepoint.
> 
> I think you are worrying about the wrong things.
> 
> I think Arjan's complaints at the KS stemmed from prior sporadic declarations on 
> lkml that there is no tracepoint ABI _at all_, and that powertop/latencytop could 
> break anytime.
> 
> But in reality i strongly disagree with such declarations, and tracepoint data that 
> is used by PowerTop/timechart/latencytop or perf is and was an ABI, simple as that - 
> and i've been enforcing that for two years. (We have so few good instrumentation 
> tools that we _really_ dont want to break them.)
> 
> At that point, realizing that we have an ABI for existing tools, i think it's 
> fundamentally misguided to go out on a limb trying to put barriers in the way of 
> other tools that do not even exist to begin with ...
> 
> Our real problem with tracing is lack of relevance, lack of utility, lack of 
> punch-through analytical power.
> 
> Trying to create a sandbox to _reduce utility_ is like the last step, and a really 
> optional step, when we have such variety that we want some control over it. It's 
> always expensive, it always reduces the tool space as collateral damage.
> 
> So please dont think of sysfs or eventfs as a tool to restrict. Think of it as a 
> tool to _organize_.
> 
> Again, i'd _LOVE_ to have the 'problem' of us having so many tools that analyze 
> application and kernel behavior in such a rich way that they use tracepoints that 
> were not supposed to be 'stable'.
> 
> I simply dont see the 'problem' that is being solved here. We had a stable ABI and 
> we didnt break sysprof or powertop/latencytop in the past and wont break it in the 
> future either.

What about a tool that picks up tracepoints that were only used by a
developer for in-field debugging, and then that tracepoint disappears
because of a design change. Is it OK for that tool to break with it?

Do all tools that use tracepoints require a "check" feature?

I guess the problem is that creators of the tools to analyze the kernel
have no idea of what they can count on and what they can't. Do we need a
process to have these tool creators request to developers to "keep this
tracepoint"?


-- Steve


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