linux-kernel - Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17

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Date:	Mon, 25 Sep 2006 13:25:04 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.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>,
	Karim Yaghmour <karim@...rsys.com>,
	Pavel Machek <pavel@...e.cz>, Joe Perches <joe@...ches.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>,
	"Jose R. Santos" <jrs@...ibm.com>
Subject: Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17

Mathieu Desnoyers wrote:
> I could declare my jump_select_label directly in assembly then.
>   

Maybe, but it could be tricky to make that label visible to C code.

>>> +call_label: \
>>> +		asm volatile ("" : : ); \
>>> +		MARK_CALL(name, format, ## args); \
>>> +		asm volatile ("" : : ); \
>>> +over_label: \
>>> +		asm volatile ("" : : ); \
>>>  
>>>       
>> These asm volatiles won't do anything at all. What are you trying to 
>> achieve?
>>     
>
> I want to make sure that the call_label's address will be exactly after the 2nd
> byte of the jump instruction. The over_label does not really matter, as long as
> it points to a correct spot in the execution flow. The most important is that
> it stays near the jump instruction.
>   

The "volatile" modifier for "asm" *only* means that the asm emitted if 
the code is reachable at all; it doesn't make any constraints about 
relative ordering of the various asm volatile statement with respect to 
each other, or with respect to other code.

> I could probably do all this in assembly too.
>   

Perhaps, though doing as much as possible visible to gcc has its 
benefits.  Tricky either way.

>>> +#ifdef CONFIG_MARKERS
>>> +#define MARK(name, format, args...) \
>>> +	do { \
>>> +		__label__ here; \
>>> +here:   	asm volatile(	".section .markers, \"a\";\n\t" \
>>> +				".long %0, %1;\n\t" \
>>> +				".previous;\n\t" : : \
>>> +			"m" (*(#name)), \
>>> +			"m" (*&&here)); \
>>>  
>>>       
>> Seems like a bad idea that MARK() can put one type of record in 
>> .markers, but MARK_JUMP and MARK_CALL can put different records in the 
>> same section? How do you distinguish them? Or are they certain to be 
>> exclusive? Either way, I'd probably put different mark records in 
>> different sections: .markers.jump, .markers.call, markers.labels. And 
>> define appropriate structures for the record types in each section.
>>
>>     
>
>
> struct __mark_marker {
>         const char *name;
>         const void *location;
>         char *select;
>         const void *jump_call;
>         const void *jump_over;
>         marker_probe_func **call;
>         const char *format;
> };
>
> is the structure which defines a complete record in the mark section. They are
> all tied to the same marker site, so I think it makes sense to keep them in the
> same record.
>   

I don't understand.  Your asms put things into the marker section with 
".long A, B, C".  Does does that correspond to this structure?

> Right, well, I wanted to keep a generic caller and try to make assumptions about
> the stack layout in the called function but if there is now way to do this, we
> can think of using the varargs in the probe.
>   

i386 is about the only architecture which uses the stack for calls by 
default.

    J
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