[lttng-dev] singleton tracepoints?

[lttng-dev] singleton tracepoints?

[Jesper Derehag]

Hi,

I have been thinking a little bit about if there is some way to both define/implement and call tracepoints in a single call.

Normally, defining tracepoints goes something along these lines:
1. Define tracepoint in tp.h?
? ? ? ? TRACE_EVENT(..)
2. Generate probes in tp.c
? ? ? ?&define TRACEPOINT_CREATE_PROBES
? ? ? ?&include "tp.h"
3. Call tracepoint in main.c
? ? ? ?tracepoint(...);

Obviously you could join points 1&2 if you use?lttng-gen-tp. It requires that you have a tracepoint definintioin file somewhere.

So the use case would then be to both "define" and call tracepoints in a single call.
I was thinking of something like this:

(Bad name, I know..)
singleton_tracepoint(TRACE_WARN, provider, name, const char*, "Something bad has happened!");


So the hard question then, how could we implement this?

I have so far thought of 4 possible approaches (and they are really not very well thought through, kind of brainstorming here):

1. Continue with the macro-magic as previously used tracepoint-event.h
? ? Cant think of any way to make this work. tracepoint-event.h hevaily relies on recursively including itself and the calling tracepoint definition.
? ? To replicate this behavior from inside a function simply wont work, or?

2. Somehow circumvent the "recursive includes from inside a function" problem by utilizing c++ function templates (or possibly lambda functions from c++11)
? ? Will likely not work either since templates are expanded during compilation and not during preprocessing. So the idea of creating a new scope
? ? within the template to create probes will not work either?

3. Re-implement the entire tracepoint callstack
? ? Getting a little bit closer, by doing that we can atleast inject trace packets with the correct formating and so on.
? ? But I dont see how one could also maintain the user workflow of beeing able to list probes and so on without having registered it?
? ??
? ? 3.1 Lazy register probes
? ? ? ? ? If probe was not registered previously (as we pass the singleton_tracepoint), register it then.
? ? 3.2 Never register singleton probes
? ? ? ? ? Make it possible to enable unknown tracepoints, perhaps it is already?

4. Extend lttng-gen-tp to also parse src-code for singleton_tracepoints and then generate probes as usual based on the definition.
? ? Could maybe be a little bit tricky since you need to do a bit of fuzzy parsing (exclude mentions of singleton_tracepoint(..) in comments and so on).
? ? But definetly the easiest (and portable) implementation, but would significantly add to build times.


Thoughts?

Regards,
Jesper