linux-kernel - Re: [PATCH tracing/kprobes 4/7] tracing/kprobes: Add event profiling support

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Date:	Mon, 14 Sep 2009 15:36:33 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH tracing/kprobes 4/7] tracing/kprobes: Add event	profiling
 support

Frederic Weisbecker wrote:
> On Mon, Sep 14, 2009 at 12:54:24PM -0400, Masami Hiramatsu wrote:
>>>> I'd like to have a dispatcher function and flags internally :)
>>>
>>>
>>> You mean kprobes that could support multiple probes?
>>> That would be a nice solution IMHO...
>>
>> Yeah, actually kprobes could support multiple probes on the
>> same point. But kprobe structure has many extensions which
>> kprobe-tracer doesn't need, e.g. post_handler/break_handler,
>> opcode, arch sprcific instructions.
>> Kretprobe consumes more memories for storing return points :(.
>>
>> Thus, if we know there are two functions to be called on the
>> same probe point, I think it is better to have a dispatcher.
>> (Especially, in this case, we can call fixed functions, so
>> it's easier way.)
>
>
> Yeah, you could union the post_handler with profile_handler
> or something like that.

No, you can't do that, because kprobes calls post_handler if
it is not NULL.

>
> It depends if kprobes may need one day to support an undeterminate
> number of probes.

Kprobes itself is supporting those multiple kprobes on the same
address. I meant that we don't need to have multiple kprobes on
the same "kprobe-tracer's event". Even if introducing a dispatcher,
kprobe-tracer can support multiple trace-event at the same location.

> Also, is the post_handler called at the same location than the normal
> probe?

No, post_handler is called after single-stepping.

> And is a post handler called even if there is no normal handler?

Yes, it is.

Hmm, I assume I have told about kprobes infrastructure, and have you
told about kprobe-tracer?:)

> There might be some of such factors that would force you to handle a
> lot of corner cases, things that you wouldn't need to worry about
> if you just had to maintain a simple rcu list of probes to call.


Anyway, I never see who are using post_handler:). I'm not sure why
it is needed...

-- 
Masami Hiramatsu

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

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

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