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

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Date:	Wed, 2 Dec 2009 11:34:32 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	rostedt@...dmis.org
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	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: trace/events: DECLARE vs DEFINE semantic

On Wed, 02 Dec 2009 14:19:38 -0500 Steven Rostedt wrote:

> On Wed, 2009-12-02 at 14:01 -0500, Mathieu Desnoyers wrote:
> > * 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.
> 
> Not really, they are almost identical. But one creates an event with the
> class, whereas the other does not. I find this quite convenient.
> 
> > > 
> > > 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
> 
> And what? Make another level of needless abstraction? That's sure to not
> confuse people.
> 
> > 
> > > 
> > > 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.
> 
> And I argue that the semantics here are not too far off to what those
> are. Yes, these macros behave differently if CREATE_TRACE_POINTS is
> declared or not, but I argue that the average (and below average) kernel
> developer is smart enough to understand this difference.
> 
> 
> > 
> > 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.
> 
> I would like to hear Andrew's comments too, as well as anyone else.
> Randy Dunlap seemed to already approve of these naming conventions, and
> he's a pretty picky person too.
> 
> Randy, do you agree that the use of DECLARE/DEFINE here is fine, or do
> you think that we should come up with a better naming. I do not want to
> add any needless abstraction layer for the sake of naming. These macros
> are confusing enough without that.

Yes, that's what I would expect to see used, although I haven't
been following this with the same level of detail that Mathieu has been.

Hopefully Andrew can chime in here also.

> Or do you (or anyone else) have a better name?



---
~Randy
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