linux-kernel - Re: [tip:perf/core] tracing: Add DEFINE_EVENT(), DEFINE_SINGLE

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Message-ID: <20091202180633.GF9710@Krystal>
Date:	Wed, 2 Dec 2009 13:06:33 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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, mhiramat@...hat.com,
	linux-tip-commits@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [tip:perf/core] tracing: Add DEFINE_EVENT(),
	DEFINE_SINGLE_EVENT() support to docbook

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Wed, 2009-12-02 at 11:27 -0500, Mathieu Desnoyers wrote:
> 
> > A few questions about the semantic:
> > 
> > Is "declare" here always only used as a declaration ? (e.g. only in
> > headers, never impacted by CREATE_TRACE_POINT ?)
> 
> Well yes it is impacted by CREATE_TRACE_POINT, but so is DECLARE_TRACE
> for that matter ;-)
> 
> The difference is that DECLARE_EVENT_CLASS will at most (with
> CREATE_TRACE_POINT) only create the functions that can be used by other
> events. It does not create an event itself. That is, it's not much
> different than making a "static inline function" except that function
> will not be static nor will it be inline ;-)
> 
> > 
> > Is "define" here always mapping to a definition ? (e.g. to be used in a
> > C file to define the class or event handling stub)
> 
> The DEFINE_* will create something that can be hooked to the trace
> points in other C files.
> 
> 
> > 
> > I feel that your DEFINE_EVENT_CLASS might actually be doing a bit more
> > than just "defining", it would actually also perform the declaration.
> > Same goes for "DEFINE_EVENT". So can you tell us a bit more about that
> > is the context of templates ?
> 
> 
> Well, the macros used by these are totally off the wall anyway :-) So
> any name we come up with will not match what the rest of the kernel does
> regardless. But we need to give something that is close.
> 
> I'm liking more the:
> 
> DECLARE_EVENT_CLASS, DEFINE_EVENT, DEFINE_EVENT_CLASS, because I think
> that comes the closest to other semantics in the kernel. That is (once
> again)
> 
> DECLARE_EVENT_CLASS - makes only the class. It does create helper
> functions, but if there's no DEFINE_EVENT that uses them, then they are
> just wasting space.
> 
> The DEFINE_EVENT will create the trace points in the C file that has
> CREATE_TRACE_POINTS defined. But it requires the helper functions
> created by a previous DECLARE_EVENT_CLASS.
> 
> DEFINE_EVENT_CLASS will do both create a EVENT_CLASS template, as well
> as a EVENT that uses the class. The name of the class is a separate
> namespace as the event. Here both the class and the event have the same
> name, but other events can use this class by referencing the name.
> 
> DEFINE_EVENT_CLASS(x, ...);
> 
> DEFINE_EVENT(x, y, ...);
> 
> The DEFINE_EVENT_CLASS will create a class x and an event x, then the
> DEFINE_EVENT will create another event y that uses the same class x.
> 
> 
> Actually, with the above, we may not need to have DECLARE_EVENT_CLASS()
> at all, because why declare a class if you don't have an event to use
> it?  But then again, you may not want the name of the class also a name
> of an event.

Hrm. I wonder if having DEFINE_EVENT_CLASS is really worth having,
considering that it really just does 2 things at once and may be
confusing.

I would have thought amongst the lines of the following as main API
(note: "SKETCH" is only a proposal. The idea is to do _not_ use
declare/define, as it's really something _different_ than what people
are expecting!)

SKETCH_EVENT_CLASS()

SKETCH_EVENT()

Which would use only DECLARE, or both DECLARE and DEFINE depending if
CREATE_TRACE_POINTS is set. I see the DECLARE/DEFINE more as the
"low-level" macros that are actually selected by CREATE_TRACE_POINTS:

DECLARE_EVENT_CLASS : only performs event class declarations (macros,
inlines...)

DECLARE_EVENT : only performs event instance declarations (macros,
inlines, ...). Depends on the DECLARE_EVENT_CLASS().

DEFINE_EVENT_CLASS : create instances of template functions.

DEFINE_EVENT : create event tracepoint functions. Depends on
DEFINE_EVENT_CLASS().

This way, it should make digging into the generation system internals
headhache-free. ;) I think we should really avoid re-using terms people
are familiar with for things that have a semantic intrincially different
than what people come to expect.

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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