linux-kernel - Re: [PATCH] Linux Kernel Markers 0.14 for 2.6.17

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Date:	Tue, 26 Sep 2006 18:27:58 -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>,
	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 0.14 for 2.6.17

* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> Mathieu Desnoyers wrote:
> >Hi,
> >
> >Constructing on Jeremy Fitzhardinge's comments about gcc optimizations, I
> >rewrote (once more) the markers mechanism so that the optimized mode does 
> >not
> >jump between two different inline asm. Instead, the optimized version uses 
> >a
> >load immediate (in assembly) that will be used by a test to decide of a 
> >branch
> >(in C).
> >  
> 
> I should have spelled out my point a bit more.  If you've got a flag 
> you're just testing, couldn't you just do:
> 
> 	if (__mark_enabled_##name)
> 		(*__mark_func)(...);
> 
> and do without the asms or the section?
> 

Because a supplementary memory read is added on the critical path with a normal
flag test. The assembly can provide an immediate value without any need of
memory read from the data section.

To change the behavior of the program, I just have to change the immediate value
in the movb instruction.

However, the non-optimized generic version does exactly this : it simply tests a
flag loaded from memory. It can be very useful on embedded systems where the
code is in read-only memory.

Regards,

Mathieu


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