linux-kernel - Re: Performance analysis of Linux Kernel Markers 0.20 for 2.6.17

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Date:	Sun, 8 Oct 2006 12:31:24 -0700 (PDT)
From:	dean gaudet <dean@...tic.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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	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: Performance analysis of Linux Kernel Markers 0.20 for 2.6.17

On Sat, 30 Sep 2006, Mathieu Desnoyers wrote:

> - Optimized
> 
> static int my_open(struct inode *inode, struct file *file)
> {
>    0:   55                      push   %ebp
>    1:   89 e5                   mov    %esp,%ebp
>    3:   83 ec 0c                sub    $0xc,%esp
>         MARK(subsys_mark1, "%d %p", 1, NULL);
>    6:   b0 00                   mov    $0x0,%al <-- immediate load 0 in al
>    8:   84 c0                   test   %al,%al
>    a:   75 07                   jne    13 <my_open+0x13>

why not replace the mov+test with "xor %eax,%eax" and then change the 0x75 
to a 0x74 to change from jne to je when you want to enable the marker?

i.e. disabled:

	31 c0	xor %eax,%eax
	75 07	jne 13

enabled:

	31 c0	xor %eax,%eax
	74 07	je 13

it would save 2 bytes, 1 instruction and avoid partial register writes... 
and still has the nice property that a single byte store into the code is 
required for enable/disable (which sounds like a great property -- i 
assume you were deliberately going for that).

i assume there's probably no reason to tie the sequence to eax either -- 
you could let gcc choose it (or maybe you already do).

-dean
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