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Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes
- From: Ingo Molnar <mingo at elte dot hu>
- To: Frederic Weisbecker <fweisbec at gmail dot com>
- Cc: Masami Hiramatsu <mhiramat at redhat dot com>, Steven Rostedt <rostedt at goodmis dot org>, lkml <linux-kernel at vger dot kernel dot org>, Thomas Gleixner <tglx at linutronix dot de>, Arnaldo Carvalho de Melo <acme at redhat dot com>, Mike Galbraith <efault at gmx dot de>, Paul Mackerras <paulus at samba dot org>, Peter Zijlstra <a dot p dot zijlstra at chello dot nl>, Christoph Hellwig <hch at infradead dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Jim Keniston <jkenisto at us dot ibm dot com>, "Frank Ch. Eigler" <fche at redhat dot com>, "H. Peter Anvin" <hpa at zytor dot com>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>
- Date: Tue, 20 Oct 2009 08:43:34 +0200
- Subject: Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes
- References: <20091017000711.16556.69935.stgit@dhcp-100-2-132.bos.redhat.com> <20091017080203.GA4155@elte.hu> <20091017103427.GA31238@elte.hu> <4ADAAF50.9040604@redhat.com> <20091019075103.GF17960@elte.hu> <20091019110055.GA5549@nowhere> <20091019112125.GA12829@elte.hu> <20091019193248.GB4880@nowhere>
* Frederic Weisbecker <fweisbec@gmail.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@2465' versus 'schedule@2555' that kind of 'where in
the function is the probe, roughly' subjective impression is lost.
Ingo