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
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