linux-kernel - Re: [PATCH 3/3] kmemtrace: Use tracepoints instead of markers.

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Message-ID: <20090105180933.GB10366@Krystal>
Date:	Mon, 5 Jan 2009 13:09:33 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] kmemtrace: Use tracepoints instead of markers.

* Eduard - Gabriel Munteanu (eduard.munteanu@...ux360.ro) wrote:
> On Mon, Jan 05, 2009 at 11:05:34AM -0500, Mathieu Desnoyers wrote:
> > Hopefully it does, especially when there are no side-effects. Can you
> > also try with -Os ?
> > 
> > Mathieu
> 
> Here's the disassembled code when using -Os. It seems it's optimised, as
> with -O2. My GCC's version is 4.3.2 (Gentoo Linux).
> 
> If you want to test yourself, the output was generated with 'objdump -d
> -S -a'.
> 
> What do you think?
> 
> static void print_that(unsigned long num)
> {
> 	printf("input << 5 == %lu\n", num);
>   40062d:	48 c1 e6 05          	shl    $0x5,%rsi
>   400631:	bf 5e 07 40 00       	mov    $0x40075e,%edi
>   400636:	31 c0                	xor    %eax,%eax
>   400638:	e8 6b fe ff ff       	callq  4004a8 <printf@plt>
> 	sscanf(argv[2], "%lu", &in);
> 
> 	call_do_something(in);
> 
> 	return 0;
> }
>   40063d:	5a                   	pop    %rdx
>   40063e:	59                   	pop    %rcx
>   40063f:	31 c0                	xor    %eax,%eax
>   400641:	5b                   	pop    %rbx
>   400642:	c3                   	retq   
> 
> 
> 	Eduard
> 

It looks good. Although I wonder if gcc will still optimize this in more
complicated functions. Just to be 100% sure, I would recommend testing
it in larger functions like schedule() in the kernel tree. But so far it
looks like it does not hurt much to leave a small supplementary operation
in the unlikely() branch.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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