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

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Message-ID: <20090914205250.GG6045@nowhere>
Date:	Mon, 14 Sep 2009 22:52:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.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

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.

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