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

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Date:	Wed, 13 Jan 2010 18:36:27 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	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 Wed, 2010-01-13 at 17:12 -0500, Masami Hiramatsu wrote:
> Masami Hiramatsu wrote:
> > 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 $*
> > 
> > I recommend you to add below line at the end of the script,
> > from my experience. :)
> > 
> > echo nop > /debug/tracing/current_tracer
> 
> Oops, my bad, it doesn't work after exec...
> But, it is very important to disable function tracer after
> tracing target process.
> 
> So, perhaps, below script may work.
> 
> #!/bin/sh
> (echo $BASHPID > /debug/tracing/set_ftrace_pid
>  echo function > /debug/tracing/current_tracer
>  exec $*)
> echo nop > /debug/tracing/current_tracer

Unfortunately, that would lose the entire trace you just recorded.

So perhaps adding:

 trace-cmd extract
 echo nop > /debug/tracing/current_tracer

would work better. The extract feature of trace-cmd pulls the data from
the kernel buffer and saves it in a file format.

-- Steve


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