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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Sep 14, 2009 at 03:36:33PM -0400, Masami Hiramatsu wrote:
> 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.


Yeah, I was meaning: having the normal probe call ftrace handler
and post probe call perf handler.
But well, that looks not that sane (your dispatch idea looks fine).


>>
>> 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.


Yeah, I was not meaning several kprobes on the same address but
actually a single kprobe instance with multiple probe callbacks.

But your dispatch patch (sorry) does that in essence, though
specialized to the ftrace/perf couple but that's fine since nothing
else may need to have multiplexed probes.


>> Also, is the post_handler called at the same location than the normal
>> probe?
>
> No, post_handler is called after single-stepping.


Ok.


>
>> 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?:)


No I was talking about kprobes in general :)



>> 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...


May be it's a legacy of code that was used by the past, in which case
that could perhaps be cleaned up?