linux-kernel - Re: [tip:perf/core] events: Rename TRACE_EVENT_TEMPLATE() to DECLARE_EVENT

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Date:	Thu, 26 Nov 2009 09:45:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, penberg@...helsinki.fi, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] events: Rename TRACE_EVENT_TEMPLATE() to
 DECLARE_EVENT_CLASS()

On Thu, 2009-11-26 at 09:40 +0100, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 

> > I would like to hear what others think about this change before we go 
> > ahead and implement it.
> 
> You mean TRACE_EVENT() -> DEFINE_SINGLE_EVENT()? Sure, we want todo it 
> in a more quiet moment of the kernel cycle, not now.
> 
> (TRACE_EVENT_TEMPLATE OTOH has existed for just a few days so it's not a 
> problem.)

Yes the template name is new, I'm not talking about that on
particularly.

> 
> > A lot of developers have just learned about TRACE_EVENT and now it 
> > just disappeared. Well, not really, but in the sense of ' find 
> > linux.git -name '*.[ch]' | xargs grep TRACE_EVENT' it no longer 
> > exists.
> 
> A second problem with the TRACE_EVENT name is that it's not just for 
> tracing - we dont necessarily 'trace' events here. We can use the event 
> callbacks to collect pure counts:

Then we might as well rename the "trace_*" all over the kernel.

> 
> | aldebaran> perf stat -e sched:sched_wakeup ./hackbench 10
> | Time: 0.093
> |
> |  Performance counter stats for './hackbench 10':
> |
> |           15481  sched:sched_wakeup      
> |
> |     0.107390574  seconds time elapsed
> 
> etc.

Right, because it hooked into a trace_point.

> 
> A third problem is that the name 'TRACE_EVENT' does not tell us what is 
> being done. Do we declare it? Do we also define it?

That's exactly the point. It does both. I actually tried to avoid the
"DEFINE/DECLARE" because it becomes confusing to what it does. The
TRACE_EVENT macros are obviously unique in the kernel. There are
"DECLARE_*" and "DEFINE_*" all over the kernel. And they have an obvious
meaning. DECLARE_* is used to set up a declaration for a header.
DEFINE_* creates the instance.  But TRACE_EVENT will default declare
event, but when CREATE_TRACE_POINTS is set, it defines the instances. Oh
we should rename that to CREATE_EVENTS?

> 
> DEFINE_SINGLE_EVENT() solves all these problems:
> 
>  - It's obvious what it does
> 
>  - It suggests users of it that there's another non-single-event 
>    facility, gently nudging them towards the use of the more efficient
>    DEFINE_EVENT_CLASS() + DEFINE_EVENT() method.
> 
>  - It fits nicely into the rest of the naming scheme.

Like I said earlier, I'm not really attached to the name. Except that
there's already a lot of documentation (I've given tutorials about it)
using the TRACE_EVENT name. But who am I to decide?

-- Steve


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