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

[Masami Hiramatsu <mhiramat@redhat.com>]

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@redhat.com