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

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Date:	Wed, 13 Jan 2010 17:12:33 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	rostedt@...dmis.org
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

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

Thanks,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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