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

From: Frederic Weisbecker <fweisbec@gmail.com>
To: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
	lkml <linux-kernel@vger.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
	Andi Kleen <ak@linux.intel.com>,
	Christoph Hellwig <hch@infradead.org>,
	"Frank Ch. Eigler" <fche@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Jason Baron <jbaron@redhat.com>,
	Jim Keniston <jkenisto@us.ibm.com>,
	"K.Prasad" <prasad@linux.vnet.ibm.com>,
	Lai Jiangshan <laijs@cn.fujitsu.com>,
	Li Zefan <lizf@cn.fujitsu.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@gmail.com>,
	systemtap <systemtap@sources.redhat.com>,
	DLE <dle-develop@lists.sourceforge.net>
Subject: Re: [PATCH tracing/kprobes 0/7] tracing/kprobes: kprobe-based event tracer update and perf support
Date: Mon, 14 Sep 2009 23:09:35 +0200	[thread overview]
Message-ID: <20090914210933.GI6045@nowhere> (raw)
In-Reply-To: <4AAEB149.2070300@redhat.com>

On Mon, Sep 14, 2009 at 05:10:33PM -0400, Masami Hiramatsu wrote:
> 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=@jiffies64: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]
>

Yeah indeed, and that fits well in the NAME=ARG:TYPE logic
that you have suggested.