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

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Message-ID: <Pine.LNX.4.64.0610081233170.28503@twinlark.arctic.org>
Date:	Sun, 8 Oct 2006 12:40:29 -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 Sun, 8 Oct 2006, dean gaudet wrote:

> 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

actually... why even destroy the register... i bet this is the best 
choice:

	39 c0	cmp %eax,%eax
	75 07	jne 13

that satisfies the macro-op fusion rules on core2 as well... now you 
shouldn't even ask gcc for a register you should just hardwire %eax so it 
doesn't get confused (probably need to tell it "cc" is modified).

-dean
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