[lttng-dev] Test if tracepoint is enabled

[David Bryant]

On 04/12/12 01:04, Mathieu Desnoyers wrote:
> * David Bryant (david.bryant at quantum.com) wrote:
>> Hi all,
>>
>> Is it feasible to have an officially supported way of determining if a
>> tracepoint is currently enabled?
>>
>> In my code I would like to test if the tracepoint is enabled, and if it
>> isn't, avoid doing expensive setup work.
>>
>> I've been doing the following:
>>
>>    if
>> (__builtin_expect(!!(__tracepoint_sample_component___message.state), 0))
>> {
>>        /* setup work */
>>        tracepoint(sample_component, message);
>>        /* cleanup work */
>>    }
>>
>> But it's weak for two reasons:
>>
>>   * The 'state' attribute goes true when the tracepoint is enabled, but
>>     stays true after the tracepoint is disabled. It only goes false when
>>     the session is destroyed.
>>   * It uses a private / unofficial API
>>
>> What I'd like is an official API that correctly evaluates whether a
>> tracepoint is enabled or not?
>>
>> Is this possible?
> Something like:
>
>     tracepoint_cond(provider, name, cond, ...)
>
> might do it.
>
> Where "cond" would be an expression (could be a statement-expression),
> e.g.:
>
> tracepoint_cond(myprovider, myevent,
>          ({
>                  a = something;
>                  z = other;
>                  a == z;
>          }),
>          a, z);
>
> So we could, in addition to perform some setup, also evaluate a
> condition before calling the tracepoint. The macro might look like
> (without the '\' at EOL):
>
> #define tracepoint_cond(provider, name, cond, ...)
>          do {
>                  if (caa_unlikely(__tracepoint_##provider##___##name.state)) {
>                          if (cond)
>                                  __tracepoint_cb_##provider##___##name(__VA_ARGS__);
>                  }
>          } while (0)
Hi Mathieu,

Your proposal seems to introduce a way of providing an additional 
condition for whether to trigger the tracepoint, ie, not only would the 
tracepoint need to be enabled but also 'cond' must be satisfied. This is 
cool, but what I want right now is simply a way of determining whether a 
tracepoint is enabled. If it isn't enabled then I won't do the 
setup/cleanup work to prepare the arguments.

I've been consulting __tracepoint_##provider##___##name.state but am 
unsatisfied because it's unofficial API and it isn't quite the right test.

Thanks,
Dave
> So in your case, a statement-expression that evaluates to "true" would
> be fine.
>
> Note that adding support for STAP_PROBEV in there might be a bit less
> straightforward, because STAP_PROBEV don't depend on the tracepoint
> state.
>
> Thoughts ?
>
> Thanks,
>
> Mathieu
>
>
>> Thanks,
>> Dave
>>
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