linux-kernel - Re: trace/events: DECLARE vs DEFINE semantic

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Message-Id: <1259850248.12870.132.camel@gandalf.stny.rr.com>
Date:	Thu, 03 Dec 2009 09:24:08 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>, akpm@...ux-foundation.org,
	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, randy.dunlap@...cle.com,
	wcohen@...hat.com, fweisbec@...il.com, tglx@...utronix.de,
	jbaron@...hat.com, linux-tip-commits@...r.kernel.org,
	Christoph Hellwig <hch@....de>
Subject: Re: trace/events: DECLARE vs DEFINE semantic

On Thu, 2009-12-03 at 09:09 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > On Thu, 2009-12-03 at 08:51 -0500, Masami Hiramatsu wrote:
> > 
> > > > Basically, we should have a:
> > > > 
> > > > kernel/sched_trace.c that includes the include/trace/events/sched.h and
> > > > does the define.
> > > > 
> > > > And the same goes for other trace points.
> > > 
> > > Hmm, I'd rather like to move it into kernel/events/ or something new
> > > sub directory, since those files will have just two lines (define and
> > > include).
> > 
> > I'm fine with a kernel/events dir.
> 
> Yep, sounds fine.
> 
> Maybe we could have separate files for:
> 
> a) event definitions
> b) class definitions

Um, because we don't add classes nor definitions for that matter in C
files. These files will just have:

#define CREATE_TRACE_POINT
#include <trace/event/x.h>
#include <trace/event/y.h>
[...]


> 
> ?
> 
> > 
> > > 
> > > >> e.g.
> > > >>
> > > >> @kernel/tracepoint.c
> > > >>  ...
> > > >>  #define CREATE_TRACE_POINTS
> > > >>  #include <trace/events/sched.h>
> > > >>  #include <trace/events/...>
> > > > 
> > > > We could do this for all that is defined in the include/trace/events.
> > > > 
> > > >>  ...
> > > >>
> > > >> @kernel/sched.c
> > > >>  ...
> > > >>  #include <trace/events/sched.h>	/* Just include events header */
> > > >>  ...
> > > >>
> > > >> @fs/ext4/super.c (no change, since it can be module)
> > > >>  ...
> > > >>  #define CREATE_TRACE_POINTS
> > > >>  #include <trace/events/ext4.h>
> > > > 
> > > > Perhaps we should move out anything in include/trace/events that is also
> > > > a module into its sub system?
> > > 
> > > Would you mean putting those headers in sub-system's directory?
> > > (e.g. fs/ext4/)
> > > In that case, a problem will happen when user want to hook those
> > > tracepoint from their module, because it is hard to find those
> > > local headers.
> > 
> > Why? Modules usually do have their own headers in their sub system.
> > 
> > OK, if a module keeps their headers global (include/linux) then sure
> > they can keep their tracepoint header in include/trace/events. But I
> > still think that the module CREATE_TRACE_POINTS code should be kept with
> > the module code itself (but in a small separate file).
> 
> I agree with Steven here: modules should come with their own trace event
> definitions, and if the trace classes they use are not available in the
> standard kernel, they should come with these trace classes definitions
> too.
> 
> A small *_trace.c file linked along with the module looks fine by me.
> 
> And please, try to re-used the already existing symbol dependency
> management already present in the kernel to deal with module dependency
> on class and dependency such as:
> 
>   module-a.ko
>     defines trace class
>   module-b.ko
>   module-c.ko
> 
>     module-b and module-c define events which depend on the trace class.
> 
> If you make the event definition depend on a symbol defined in module-a,
> everything should work flawlessly. It also works if the class is defined
> in the core kernel.

I think the issue is where to find the headers.

But this does bring up another point. I don't think I designed the class
macro to be used by events in other headers. Looking at the code, since
all the shared functions are "static" it wont work.

I guess I can modify it to be global, and also export them as GPL.

-- Steve


-- Steve


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