linux-kernel - Re: [PATCH -tip -v10 7/7] tracing: add kprobe-based event tracer

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Date:	Tue, 07 Jul 2009 17:42:28 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	kvm <kvm@...r.kernel.org>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH -tip -v10 7/7] tracing: add kprobe-based event tracer

Frederic Weisbecker wrote:
> On Tue, Jul 07, 2009 at 05:31:25PM -0400, Masami Hiramatsu wrote:
>> Frederic Weisbecker wrote:
>>> On Tue, Jul 07, 2009 at 04:42:32PM -0400, Masami Hiramatsu wrote:
>>>> Frederic Weisbecker wrote:
>>>>> On Tue, Jul 07, 2009 at 03:55:28PM -0400, Masami Hiramatsu wrote:
>>>>>> Frederic Weisbecker wrote:
>>>>>>>> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
>>>>>>>> index 206cb7d..65945eb 100644
>>>>>>>> --- a/kernel/trace/trace.h
>>>>>>>> +++ b/kernel/trace/trace.h
>>>>>>>> @@ -45,6 +45,8 @@ enum trace_type {
>>>>>>>>  	TRACE_POWER,
>>>>>>>>  	TRACE_BLK,
>>>>>>>>  	TRACE_KSYM,
>>>>>>>> +	TRACE_KPROBE,
>>>>>>>> +	TRACE_KRETPROBE,
>>>>>>>>  
>>>>>>>>  	__TRACE_LAST_TYPE,
>>>>>>>>  };
>>>>>>>> @@ -227,6 +229,22 @@ struct trace_ksym {
>>>>>>>>  	char			ksym_name[KSYM_NAME_LEN];
>>>>>>>>  	char			p_name[TASK_COMM_LEN];
>>>>>>>>  };
>>>>>>>> +#define TRACE_KPROBE_ARGS 6
>>>>>>>> +
>>>>>>>> +struct kprobe_trace_entry {
>>>>>>>> +	struct trace_entry	ent;
>>>>>>>> +	unsigned long		ip;
>>>>>>>> +	int			nargs;
>>>>>>>> +	unsigned long		args[TRACE_KPROBE_ARGS];
>>>>>>> I see that you actually make use of arg as a dynamic sizeable
>>>>>>> array.
>>>>>>> For clarity, args[TRACE_KPROBE_ARGS] could be args[0].
>>>>>>>
>>>>>>> It's just a neat and wouldn't affect the code nor the data
>>>>>>> but would be clearer for readers of that code.
>>>>>> Hmm. In that case, I think we'll need a new macro for field
>>>>>> definition, like TRACE_FIELD_ZERO(type, item).
>>>>>
>>>>> You mean that for trace_define_field() to describe fields of events?
>>>>> Actually the fields should be defined dynamically depending on how
>>>>> is built the kprobe event (which arguments are requested, how many,
>>>>> etc..).
>>>> Yeah, if you specified a probe point with its event name, the tracer
>>>> will make a corresponding event dynamically. There are also anonymous
>>>> probes which don't have corresponding events. For those anonymous
>>>> probes, I need to define two generic event types(kprobe and kretprobe).
>>>>
>>>> Thank you,
>>>
>>> Ok. Btw, why do you need to define those two anonymous events?
>>> Actually your event types are always dynamically created.
>>> Those you defined through TRACE_FORMAT_EVENT are only "ghost events",
>>> they only stand there as a abstract pattern, right?
>>>
>> Not always created.
>>
>> Below command will create an event "event1";
>> p probe_point:event1 a1 a2 a3 ... > /debug/tracing/kprobe_events
>>
>> But next command doesn't create.
>> p probe_point a1 a2 a3 ... > /debug/tracing/kprobe_events
> 
> 
> Aah, ok.
> 
> 
>> This just inserts a kprobe to probe_point. the advantage of this
>> "simple" command is that you never be annoyed by making different
>> name for new events :-)
> 
> 
> Indeed.
> But speaking about that, may be you could dynamically create a name
> following this simple model: func+offset
> Unless we can set several kprobes on the exact same address?

Actually, we can...
I thought that someone might want to insert events in the same
address for retrieving more than 6 arguments.

Thanks,

-- 
Masami Hiramatsu

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

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

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