linux-kernel - Re: Efficient x86 and x86

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Message-ID: <20080813184142.GM1366@one.firstfloor.org>
Date:	Wed, 13 Aug 2008 20:41:42 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: Efficient x86 and x86_64 NOP microbenchmarks

> So microbenchmarking this way will probably make some things look 
> unrealistically good. 

Must be careful to miss the big picture here.

We have two assumptions here in this thread:

- Normal alternative() nops are relatively infrequent, typically
in points with enough pipeline bubbles anyways, and it likely doesn't
matter how they are encode. And also they don't have an issue
with mult part instructions anyways because they're not patched
at runtime, so always the best known can be used.

- The one case where nops are very frequent and matter and multipart
is a problem is with ftrace noping out the call to mcount at runtime 
because that happens on every function entry.
Even there the overhead is not that big, but at least measurable 
in kernel builds.

Now the numbers have shown that just by not using frame pointer (
-pg right now implies frame pointer) you can get more benefit 
than what you lose from using non optimal nops.

So for me the best strategy would be to get rid of the frame pointer
and ignore the nops. This unfortunately would require going away
from -pg and instead post process gcc output to insert "call mcount"
manually. But the nice advantage of that is that you could actually 
set up a custom table of callers built in a ELF section and with
that you don't actually need the runtime patching (which is only
done currently because there's no global table of mcount calls),
but could do everything in stop_machine(). Without
runtime patching you also don't need single part nops. 

I think that would be the best option. I especially like it because
it would prevent forcing frame pointer which seems to be costlier
than any kinds of nosp.

-Andi

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