linux-kernel - Re: [PATCH -tip perf/probes 0/5] perf-probe and kprobe-tracer updates

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Date:	Wed, 04 Nov 2009 09:11:33 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jason Baron <jbaron@...hat.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [PATCH -tip perf/probes 0/5] perf-probe and kprobe-tracer	updates

Frederic Weisbecker wrote:
> On Tue, Nov 03, 2009 at 07:12:04PM -0500, Masami Hiramatsu wrote:
>> BTW, I think perf-probe and kprobe-event might better share
>> similar syntax for not confusing users. And for that purpose,
>> perf-probe syntax should introduce event/group specifier,
>> for example,
>
>
> I personally more imagine the debugfs kprobe-event interface as
> something used by higher level applications rather than users.
>
> I've tried to use kprobe events directly by the past to do
> some debugging, and once I wanted to go further a simple function
> probe, like fetching a variable or putting a probe in a given branch,
> it rapidly grew into a pain: I had to read assembly code, guess
> which register was matching which variable, etc... It works, but
> it takes too much time, and printk() rapidly becomes a temptation :)
>
> It too low-level, but its use through perf brings all that to the
> human dimension.
>
> So, I'm not sure we really need to have such tight syntax between
> both, since the debugfs more likely behaves as a gateway, something
> I don't imagine to be used broadly as an end-user interface but mostly
> as a kernel interface.

I see, and I also found that the syntax never be same, since
perf-probe doesn't need argument names etc. kprobe_events
interface may be mostly for higher level scripts or programs.

> Especially we shouldn't break the perf probe syntax simplicity
> just because we want both syntaxes to be tight.

Agreed. OK, so let it be :-)

> (Nothing related to the event/group feature itself, it's just an
> opinion about the need of this similarity between two interfaces).
>
>
>>   perf probe "newgroup:newevnt=func:10 arg1 arg2"
>>
>> adds the newevent under newgroup. On the other hand, ftrace
>> users can also add a new event as below;
>>
>>   echo 'newgroup:newevent=func+0x18 arg1=$a1 arg2=$a2'>  kprobe_events
>>
>> Any thoughts?
>
>
> Yeah, that would probably be nice, especially once we have a good
> collection of probes to handle and to organize in a sensical output.
>
> But it would be better to have that as an optional thing:
>
> 	perf probe "[group:name=]func...."


Sure, of course it should be optional. :-)

> so that we keep the simplicity of:
>
> 	perf probe func
>
> I guess you meant it as optional already, but just in case... :)

Thank you :-)

-- 
Masami Hiramatsu

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

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

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