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

[Masami Hiramatsu]

Frederic Weisbecker wrote:
> On Mon, Sep 14, 2009 at 12:54:24PM -0400, Masami Hiramatsu wrote:
> > > > 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.)
>
>
> Yeah, you could union the post_handler with profile_handler
> or something like that.

No, you can't do that, because kprobes calls post_handler if
it is not NULL.

> It depends if kprobes may need one day to support an undeterminate
> number of probes.

Kprobes itself is supporting those multiple kprobes on the same
address. I meant that we don't need to have multiple kprobes on
the same "kprobe-tracer's event". Even if introducing a dispatcher,
kprobe-tracer can support multiple trace-event at the same location.

> Also, is the post_handler called at the same location than the normal
> probe?

No, post_handler is called after single-stepping.

> And is a post handler called even if there is no normal handler?

Yes, it is.

Hmm, I assume I have told about kprobes infrastructure, and have you
told about kprobe-tracer?:)

> There might be some of such factors that would force you to handle a
> lot of corner cases, things that you wouldn't need to worry about
> if you just had to maintain a simple rcu list of probes to call.


Anyway, I never see who are using post_handler:). I'm not sure why
it is needed...

--
Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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