[lttng-dev] Test if tracepoint is enabled

[Woegerer, Paul]

On 12/06/2012 02:19 AM, David Bryant wrote:
> statement-expression would also work. Are we conflating separate ideas -
> the idea of user conditions and the idea of setup/cleanup?

FWIW, tracepoint_cond could be implemented as loan pattern (in pseudocode):

with_tp( provider, name, REF ) {
  if (cond) {
     // pre emit code block
     ...

     emit_REF( arg0, ... );

     // post emit code block
     ...
  }
}

This could even be nested. E.g.

with_tp( provider1, nameA, REF1 ) {
  with_tp( provider2, nameB, REF2 ) {
    if (cond) {
       // pre emit code block if both are enabled
       ...

       emit_REF1( arg0x, ... );
       emit_REF2( arg0y, ... );

       // post emit code block if both are enabled
       ...
    }
  }
}

Nesting would be rather unpleasant with the proposed
tracepoint_cond(provider, name, pre, post, ...).

--
Paul

> 
>> While we are there, it would be good to make the "pre" handler test a
>> condition. For your use-case, you would just have to pass a pre :
>>
>>   { printf("pre\n");  a = malloc(16); 1; }
>>
>> So we could do:
>>
>>     if (pre)
>>          __tracepoint_cb_##provider##___##name(__VA_ARGS__);
>>     (post)
>>
>> and therefore support not only to setup something, but also to add an
>> extra check on whether tracing should be skipped or not. If someone
>> passes "1" as end of the compound statement, it will always evaluate to
>> 1, and therefore the compiler will remove any extra branch in -O2.
>>
>> One question stands though: how would we integrate this with sdt.h
>> STAP_PROVEV() ?
> I'm yet to look at this.
> 
> Thanks,
> Dave
> 
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Paul Woegerer | SW Development Engineer
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