linux-kernel - Re: [PATCH] Linux Kernel Markers 0.7 for 2.6.17 (with type checking!)

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Date:	Fri, 22 Sep 2006 11:27:37 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Martin Bligh <mbligh@...gle.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, 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>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers 0.7 for 2.6.17 (with type checking!)

* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> Mathieu Desnoyers wrote:
> >* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> >  
> >>Mathieu Desnoyers wrote:
> >>    
> >>>#define MARK_SYM(name) \
> >>>       do { \
> >>>               __label__ here; \
> >>>               volatile static void *__mark_kprobe_##name \
> >>>                       asm (MARK_CALL_PREFIX#name) \
> >>>                       __attribute__((unused)) = &&here; \
> >>>here: \
> >>>               do { } while(0); \
> >>>       } while(0)
> >>>
> >>>Which fixes the problem. Some tests showed me that the compiler does not 
> >>>unroll
> >>>an otherwise unrolled loop when this specific macro is called. (test 
> >>>done with
> >>>-funroll-all-loops).
> >>>      
> >>Eh?  I thought you wanted to avoid changing the generated code?  
> >>Inhibiting loop unrolling could be a pretty large change...
> >>
> >>    
> >
> >Yes, if possible. But letting gcc duplicate those symbols brings many 
> >questions,
> >such as : how can we name each of them differently ? Is there any way to
> >automatically increment an "identifier" counter in assembly ?
> 
> Use a section instead:
> 
> struct marker {
> 	const char *name;
> 	const void *location;
> };
> 
> #define MARKER_SYM(name)
> 	do {
> 		__label__ here;
> 	here:	asm volatile(".section \".markers\"; .long %0, %1; 
> 	.previous" : : "m" (#name), "m" (*&&here));\
> 	} while(0);
> 
> Not a linker symbol, but it does let you find all the places containing 
> a particular mark.
> 

Very clever idea, as it lessens the impact on the compiler optimisations. Any
ideas about how we could fit in a list of "read" memory constraints based on a
vargs list in the macro ?

Mathieu

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