linux-kernel - Re: tracepoint maintainance models

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Message-ID: <20060918032120.GA13076@elte.hu>
Date:	Mon, 18 Sep 2006 05:21:20 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Karim Yaghmour <karim@...rsys.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jes Sorensen <jes@....com>,
	Andrew Morton <akpm@...l.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>,
	"Martin J. Bligh" <mbligh@...igh.org>
Subject: Re: tracepoint maintainance models


Hi,

* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Ingo Molnar (mingo@...e.hu) wrote:
> > Karim, i dont usually reply if you insult me (and you've grown a habit 
> > of that lately ), but this one is almost parodic. To understand my 
> > point, please consider this simple example of a static in-source markup, 
> > to be used by a dynamic tracer:
> > 
> >   static int x;
> > 
> >   void func(int a)
> >   {
> >        ...
> >        MARK(event, a);
> >        ...
> >   }
> > 
> > if a dynamic tracer installs a probe into that MARK() spot, it will have 
> > access to 'a', but it can also have access to 'x'. While a static 
> > in-source markup for _static tracers_, if it also wanted to have the 'x' 
> > information, would also have to add 'x' as a parameter:
> > 
> > 	MARK(event, a, x);
> > 
> 
> Hi,
>
> If I may, if nothing marks the interest of the tracer in the "x" 
> variable, what happens when a kernel guru changes it for y (because it 
> looks a lot better). The code will not compile anymore when the markup 
> marks the interest for x, when your "dynamic tracer" markup will 
> simply fail to find the information. My point is that the markup of 
> the interesting variables should follow code changes, otherwise it 
> will have to be constantly updated elsewhere (hmm ? Documentation/ 
> someone ?)

yeah - but it shows (as you have now recognized it too) that even static 
markup for dynamic tracers _can_ be fundamentally different, just 
because dynamic tracers have access to information that static tracers 
dont.

(Karim still disputes it, and he is still wrong.)

> I would say that not marking a static variable just because it is less 
> visually intrusive is a not such a good thing to do. That's not 
> because we *can* that we *should*.

yeah. But obviously the (small but present) performance advantage is 
there too, so it shouldnt be rejected out of hand. If a parameter is not 
mentioned then it does not have to be prepared for function paramter 
passing, etc. So it's 1-2 instructions less. So if this is in some 
really stable area of code then it's a valid optimization.

	Ingo
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