linux-kernel - Re: [PATCH tracing/kprobes 0/7] tracing/kprobes: kprobe-based event tracer update and perf support

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Date:	Mon, 14 Sep 2009 17:10:33 -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>,
	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>,
	Jim Keniston <jkenisto@...ibm.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>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [PATCH tracing/kprobes 0/7] tracing/kprobes: kprobe-based	event
 tracer update and perf support

Frederic Weisbecker wrote:
> On Mon, Sep 14, 2009 at 01:16:13PM -0400, Masami Hiramatsu wrote:
>> Frederic Weisbecker wrote:
>>> On Fri, Sep 11, 2009 at 12:03:30PM -0400, Masami Hiramatsu wrote:
>>>> Frederic Weisbecker wrote:
>>>>> May be another step in the todo-list that would be nice: define the format
>>>>> for a type. Like it's done from ftrace events.
>>>>
>>>> Thanks!
>>>>
>>>> BTW, I'm not sure what the type means. Each event already has its own
>>>> event ID and event_call. Could you tell me which part of ftrace I should
>>>> refer to ?
>>>>
>>>
>>>
>>> Actually I meant the format for a field.
>>> Say you define filename=arg1, it would be nice to have
>>>
>>> 	print "%s", filename
>>>
>>> in the format file.
>>
>> Ah, indeed. It is better to support 'type' casting for each argument.
>> I think type casting can be done as below syntax.
>>
>> NAME=ARG:TYPE
>> e.g.
>> jiffies64=@...fies64:u64
>> message=%ax:str
>>
>
>
> Yeah looks good!
>
>
>
>>> Hmm, now that I think about it, we can't dereference an array...for now :-)
>>
>> :-)
>> BTW, currently, an entry of the array can be shown, e.g. +10(sa).
>> Hmm, for more complex dereference(e.g. accessing a->b[a->c]), it might need
>> another dereferencing syntax(e.g. "sa[16][sa[8]]"), or
>> just allow to use braces(e.g. "+(+8(sa))(+16(sa))").
>
>
> Well, that may be too much complexity.
> I guess if we want multi level dereference, say you want a->b->c
> it should be sufficient to probe the point where b->c gets it's
> value (if any).
>
> But it would be nice to fetch a range: sa[begin:end]
> Or at least just giving the length of the range.

Hmm, I think it is better to treat the range as a type, because
we don't know how to express each entry without the type.

NAME=ARG:TYPE[LEN]
e.g.
regs=sa:u64[16]

Thank you,

-- 
Masami Hiramatsu

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

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

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