linux-kernel - Re: [PATCH 03/13] jump label v9: x86 support

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Message-ID: <20100610132616.GA19561@basil.fritz.box>
Date:	Thu, 10 Jun 2010 15:26:16 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, mhiramat@...hat.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, vgoyal@...hat.com,
	sam@...nborg.org
Subject: Re: [PATCH 03/13] jump label v9: x86 support

On Thu, Jun 10, 2010 at 02:14:40PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, 2010-06-09 at 17:39 -0400, Jason Baron wrote:
> > > +       select HAVE_ARCH_JUMP_LABEL if !CC_OPTIMIZE_FOR_SIZE
> > 
> > That deserves a comment somewhere, it basically makes OPTIMIZE_FOR_SIZE 
> > useless...
> 
> Hm, we need more than a comment for that - distros enable CC_OPTIMIZE_FOR_SIZE 
> all the time, for the massive kernel image (and hotpath cache footprint) 
> savings. Is this fixable?

Actually the big distros (RHEL, SLES) disable it all the time now. It costs you 
in some benchmarks. The code generated by -Os is often terrible.

Nearly everytime I investigate some bad asm code being generated by gcc
it goes away when that flag is disabled.

A much better to get smaller kernel images is to do more __cold
annotations for slow paths. Newer gcc will then simply only do -Os for these 
functions.

It's already done for __init.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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