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

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Message-ID: <20091202190135.GA23316@Krystal>
Date:	Wed, 2 Dec 2009 14:01:35 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>, akpm@...ux-foundation.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: trace/events: DECLARE vs DEFINE semantic

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Wed, 2009-12-02 at 13:06 -0500, Mathieu Desnoyers wrote:
> > *
> > 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.
> 
> We keep it because that's what TRACE_EVENT currently is. It would suck
> to have to replace every TRACE_EVENT there is now with a
> DECLARE_EVENT_CLASS and DEFINE_EVENT. Although this would push
> developers into using classes.

I agree that keeping something for backward compatibility is good, but
what I dislike the most is the similarity between the
DECLARE_EVENT_CLASS and DEFINE_EVENT_CLASS which have completely
unrelated semantics. This is really misleading.

> 
> > 
> > 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.
> 
> Egad No! It would make it a living nightmare. The internals reuse the
> define macro, and there's no intermediate. By changing the
> DECLARE_EVENT_CLASS to another name (SKETCH_EVENT_CLASS) we would have
> to add something like this:
> 
> #define SKETCH_EVENT_CLASS(name, proto, args, tstruct, print) \
> 	DECLARE_EVENT_CLASS(name, PARAMS(proto), PARAMS(args),\
> 		PARAMS(tstruct), PARAMS(print))
> 
> We don't have a intermediate or "low level" macro in use here. Whatever
> we give to the user is what we use.
> 

Maybe we should consider having one. e.g.:

#ifdef CREATE_TRACE_POINTS

SKETCH_EVENT_CLASS maps to DEFINE_EVENT_CLASS

#else

SKETCH_EVENT_CLASS maps to DECLARE_EVENT_CLASS

#endif

> 
> I think the kernel developers are smart enough to figure out that these
> macros are not a typical DECLARE/DEFINE that is elsewhere. But I think
> using the DECLARE/DEFINE names will give them a better idea of what is
> happening than to make up something completely new.

In my opinion, re-using a well-known keyword (e.g. DECLARE/DEFINE) but
applying a different semantic to what is generally agreed upon is a
recipe for confusing developers and users, who will skip the review of
some pieces of code assuming they already know what "DECLARE" and
"DEFINE" stands for.

I argue here that the content of trace/events/ headers are _not_ per se
declarations nor definitions, and hence they should not confuse people
by using inappropriately well-known keywords. They are actually more
evolved macros that can be turned in either a declaration or definition,
depending if CREATE_TRACE_POINTS is declared.

When I created the markers/tracepoints, Andrew Morton explained to me
the importance of distinguishing DECLARE vs DEFINE macros. I would
really like to hear his point of view on the current question.

Thanks,

Mathieu


> 
> -- Steve
> 

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