linux-kernel - Re: tracepoint maintainance models

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Date:	Fri, 06 Oct 2006 01:33:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Vara Prasad <prasadav@...ibm.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Paul Mundt <lethal@...ux-sh.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Andrew Morton <akpm@...l.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.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>,
	systemtap <systemtap@...rceware.org>
Subject: Re: tracepoint maintainance models

Coming into this really late, and I'm still behind in reading this and
related threads, but I want to throw this idea out, and it's getting
late.

On Mon, 2006-09-18 at 13:28 -0700, Vara Prasad wrote:
> Alan Cox wrote:
> 
> >
> >>This still doesn't solve the problem of compiler optimizing such that a 
> >>variable i would like to read in my probe not being available at the 
> >>probe point.
> >>    
> >>
> >
> >Then what we really need by the sound of it is enough gcc smarts to do
> >something of the form
> >
> >	.section "debugbits"
> >	
> >	.asciiz 'hook_sched'
> >	.dword l1	# Address to probe
> >	.word 1		# Argument count
> >	.dword gcc_magic_whatregister("next"); [ reg num or memory ]
> >	.dword gcc_magic_whataddress("next"); [ address if exists]
> >
> >
> >Can gcc do any of that for us today ?
> >
> >  
> >
> No, gcc doesn't do that today.
> 
> 


---- cut here ----
#include <stdio.h>

#define MARK(label, var)			\
	asm ("debug_" #label ":\n"		\
	     ".section .data\n"			\
	     #label "_" #var ": xor %0,%0\n"	\
	     ".previous" : : "r"(var))


static int func(int a)
{
	int y;
	int z;

	y = a;
	MARK(func, y);
	z = y+2;

	return z;

}



static void read_label(void)
{
	extern unsigned short regA;
	unsigned short *r = &regA;
	char *regs[] = {
		"A", "B", "C", "D", "DI", "BP", "SP", "CH"
	};
	int i;
	extern unsigned short func_y;
	extern unsigned long debug_func;

	asm (".section .data\n"
	     "regA: xor %eax,%eax\n"
	     "regB: xor %ebx,%ebx\n"
	     "regC: xor %ecx,%ecx\n"
	     "regD: xor %edx,%edx\n"
	     "regDI: xor %edi,%edi\n"
	     "regBP: xor %ebp,%ebp\n"
	     "regSP: xor %esp,%esp\n"
	     ".previous");

	for (i=0; i < 7; i++) {
		if (r[i] == func_y)
			break;
	}
	if (i < 7)
		printf("func y is in reg %s at %p\n",
		       regs[i],
		       &debug_func);
	else
		printf("func y not found!\n");
}

int main (int argc, char **argv)
{
	int g;
	g = func(argc);
	read_label();
	return g;
}
---- end cut ----

$ gcc -O2 -o mark mark.c
$ ./mark
func y is in reg B at 0x80483ce



Now the question is, isn't MARK() in this code a non intrusive marker?

So couldn't a kprobe be set at "debug_func" and we can find what
register "y" is without adding any overhead to the code being marked?

Obviously, this would need to be done special for each arch.

-- Steve


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