linux-kernel - Re: [RFC] [PATCH 7/7] Ftrace plugin for Uprobes

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Message-ID: <20100112054422.GM5243@nowhere>
Date:	Tue, 12 Jan 2010 06:44:23 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	utrace-devel <utrace-devel@...hat.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 7/7] Ftrace plugin for Uprobes

On Tue, Jan 12, 2010 at 12:08:53AM -0500, Steven Rostedt wrote:
> On Tue, 2010-01-12 at 05:54 +0100, Frederic Weisbecker wrote:
> 
> > Now what if I want to launch ls and want to profile a function
> > inside. What can I do with a trace event. I can't create the
> > probe event based on a pid as I don't know it in advance.
> > I could give it the ls cmdline and it manages to activate
> > on the next ls launched. This is racy as another ls can
> > be launched concurrently.
> 
> You make a wrapper script:
> 
> #!/bin/sh
> <add probe to ls with pid> $$
> exec $*
> 
> I do this all the time to limit the function tracer to a specific app.
> 
> #!/bin/sh
> echo $$ > /debug/tracing/set_ftrace_pid
> echo function > /debug/tracing/current_tracer
> exec $*
> 
> 
> The exec will cause the ls to have the pid of $$.


Sounds like a good idea. In this case we could indeed
think about a trace event.

It would typically have the benefit to have the same
interface than kprobes.

We can use it with perf, the only constraint is that we need
to launch the record right after creating the trace event.
Or we can pre-create them and set the pid of the target
later when we launch perf record.

And we need an enable on exec option in the probe definition.

That's a nice option.

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