linux-kernel - Re: [rfc] x86: optimise page fault path a little

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Message-ID: <alpine.LFD.2.00.0811131803300.3468@nehalem.linux-foundation.org>
Date:	Thu, 13 Nov 2008 18:07:45 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] x86: optimise page fault path a little

On Fri, 14 Nov 2008, Nick Piggin wrote:
> 
> True, but is it any better to jam them all into a 300 line function
> with gotos?

That wasn't what I was saying.

Theere are two "good" cases:

 - don't mess with things.

   This is good. Stability is good.

 - Clearly improve things. 

   This is great.

And I'll happily do either of the above.

Your patch had some improvement, but it had some clear not-so-improved 
parts. That makes it INFERIOR to just leaving things well alone.

The thing is, I'm not very much interested in just a micro-optimization 
that seems to be all about just gcc code generation. Long-term, that's 
just bad.

But if it's a clear and undeniable _cleanup_, then long-term, it's 
actually a win. If it also happens to fix some gcc stack allocation issues 
etc, then that is just gravy.

See my point? Cleanup is good. But it had better _be_ a cleanup. Random 
micro-optimization is not so good, especially not if it them makes the 
code do things that good code simply shouldn't be doing.

As it is, I don't think your patch is appropriate to be merged. 

			Linus
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