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

[Frederic Weisbecker <fweisbec@gmail.com>]

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?