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

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Date:	Tue, 7 Jul 2009 22:58:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.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

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?

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