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

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Date:	Thu, 10 Jun 2010 17:37:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
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, roland@...hat.com, rth@...hat.com,
	mhiramat@...hat.com, fweisbec@...il.com, avi@...hat.com,
	davem@...emloft.net, vgoyal@...hat.com, sam@...nborg.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 03/13] jump label v9: x86 support

* Andi Kleen <andi@...stfloor.org> wrote:

> > 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. [...]

Not all the big distros, Fedora certainly doesnt.

> [...] It costs you in some benchmarks. [...]

Microbenchmarks mostly, see below.

> The code generated by -Os is often terrible.

But it results in a kernel .text that is ~30% smaller, so considering the fact 
that most real-life kernel code execution is instruction-cache-cold it's 
generally a win.

But micro-benchmarks are instruction-cache-hot so that's where the pressure to 
remove the flag comes from.

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

That is not fixing anything, it is working bugs around.

> 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.

That's an opt-in method and we cannot reach the kinds of 30% code size 
reductions that -Os can achieve. Most code in the kernel is not cache-hot, 
even on microbenchmarks.

A much better model would be to actively mark hot codepaths with a __hot 
attribute instead. Then the code size difference can be considered on a case 
by case basis.

And where GCC produces indefensibly crap code there GCC needs to be fixed. 
Crap code often increases size so the fix would increase the efficiency of 
-Os.

Thanks,

	Ingo
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