linux-kernel - Re: tracepoint maintainance models

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Date:	Mon, 18 Sep 2006 00:26:52 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
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

* Ingo Molnar (mingo@...e.hu) wrote:
> > >   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.)

The following example voids your example : there are ways to implement static
markers that *could* have access to those variables. (implementation detail)

int x = 5;

#define MARK(a) printk(a, x)

voi func(int a)
{
  ...
  MARK(a);
  ...
}

Mathieu


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