Re: [PATCH tracing/kprobes 4/7] tracing/kprobes: Add event profilingsupport

[Masami Hiramatsu]

Frederic Weisbecker wrote:
> On Fri, Sep 11, 2009 at 12:22:16PM -0400, Masami Hiramatsu wrote:
> > Frederic Weisbecker wrote:
> > > > +static int probe_profile_enable(struct ftrace_event_call *call)
> > > > +{
> > > > +	struct trace_probe *tp = (struct trace_probe *)call->data;
> > > > +
> > > > +	if (atomic_inc_return(&call->profile_count))
> > > > +		return 0;
> > > > +
> > > > +	if (probe_is_return(tp)) {
> > > > +		tp->rp.handler = kretprobe_profile_func;
> > > > +		return enable_kretprobe(&tp->rp);
> > > > +	} else {
> > > > +		tp->rp.kp.pre_handler = kprobe_profile_func;
> > > > +		return enable_kprobe(&tp->rp.kp);
> > > > +	}
> > > > +}
> > >
> > >
> > >
> > > May be I misunderstood but it seems that concurrent uses of
> > > ftrace and perf would really mess up the result, as one would
> > > overwrite the handler of the other.
> > >
> > > Even though it's hard to imagine one using both at the same
> > > time on the same probe, but still...
> >
> > Oops, it's my misunderstanding. I thought those are exclusively
> > enabled each other.
>
>
> It's automatically managed with events because ftrace and
> and perf have their individual tracepoint probes, because
> tracepoints support multiple probes.
>
>
> > > Is it possible to have two kprobes having the exact same
> > > properties? (pointing to the same address, having the same
> > > probe handlers, etc...)
> > >
> > > Another solution would be to allow kprobes to have multiple
> > > handlers.
> >
> > It could be to have multiple kprobes on same point, but I think
> > it's waste of the memory and time in this case.
>
>
> Yeah.
>
>
> > I'd like to have a dispatcher function and flags internally :)
>
>
> You mean kprobes that could support multiple probes?
> That would be a nice solution IMHO...

Yeah, actually kprobes could support multiple probes on the
same point. But kprobe structure has many extensions which
kprobe-tracer doesn't need, e.g. post_handler/break_handler,
opcode, arch sprcific instructions.
Kretprobe consumes more memories for storing return points :(.

Thus, if we know there are two functions to be called on the
same probe point, I think it is better to have a dispatcher.
(Especially, in this case, we can call fixed functions, so
it's easier way.)

Thank you,

--
Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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