linux-kernel - Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes

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Message-ID: <20091020064334.GK8550@elte.hu>
Date:	Tue, 20 Oct 2009 08:43:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf
	probe and kprobe-tracer bugfixes


* Frederic Weisbecker <fweisbec@...il.com> wrote:

[...]
>
> I think absolute and relative line modes are not colliding/contending 
> at all but actually fit two different needs.

Definitely so.

> - absolute is nice when you are lonely doing kernel debugging.
>   (can be expanded at will once you imagine user probes)
>   You are stuck in your code editor, trying to figure out the
>   origin of your problem and then you think it would be nice
>   to set a probe in branch 1 and in branch 2 inside func_foo().
>   Then you already have absolute lines and relying in
>   perf probe --list func_foo() to resolve an absolute line into
>   a relative one is a very undesired middle step.

Of course - absolute numbers definitely rule for everything that works 
on a whole-file basis. (I'd argue that if you do that from an editor 
then you want a short macro that just sets a probe there - much like a 
breakpoint. Such an editor macro would want to use absolute numbers.))

> - relative is nice in some other cases. When you already have
>   the function target in mind, you even don't need to check your
>   editor, just a quick check to this command and get the relative
>   line. But also when you want to transmit a probe reference
>   in a mailing list because of its better lifetime.

also useful for command line workflows: 'perf probe --list' output - i 
think we users to generate func_symbol+rel_position kind of probes.

Plus a relative position is more intuitive as well. If you see 
'schedule+10' versus 'schedule+102', you'll know it immediate that the 
first one is early in the function while the second one is near the end.

If you see 'schedule@...5' versus 'schedule@...5' that kind of 'where in 
the function is the probe, roughly' subjective impression is lost.

	Ingo
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