linux-kernel - Re: Linux 2.6.32-rc1

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Message-ID: <20090930185315.GA9716@elte.hu>
Date:	Wed, 30 Sep 2009 20:53:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Linux 2.6.32-rc1


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, 30 Sep 2009, Eric Dumazet wrote:
> > 
> > lock; cmpxchg8b (%%esi)
> > 
> > gives 4 bytes opcode : f0 0f c7 0e
> > Because alternative (call cmpxchg8b_emu) uses 5 bytes, a nop will be added.
> > 
> > Choosing ".byte 0xf0, 0x0f, 0xc7, 0x4e, 0x00"  aka "lock cmpxchg8b 0x0(%esi)" is a litle bit better ?
> 
> And if you want to be really clever, you would want to handle the 
> non-SMP case too, where you have just "cmpxchgb (%%esi)" (three bytes) 
> without the lock prefix.
> 
> However, at this point I think Arjan's patch is already way superior 
> to what we have now (feel free to take a look at what we generate on 
> 32-bit without PAE today - just have a barf-bag handy), so all I'd 
> really want is a few "tested-by"s to make me feel happier about it, 
> and a few more people looking at the emulation routine to all say "ok, 
> looks sane, ACK".
> 
> And at that point we can then either make "cmpxchg()" just do the 
> 8-byte case natively, or just take your patch to change sched_clock.c 
> to now use the no-longer-entirely-disgusting cmpxchg64().
> 
> Ingo - I suspect both those patches should just go through you. You do 
> both x86 and scheduler, so I'll happily pull the end result.

Yeah - working on it - just got back from a trip. It's two risky patches 
and if that place breaks everyone will be affected so i'll probably send 
the pull request tomorrow.

	Ingo
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