linux-kernel - Re: [PATCHSET percpu#for-next] percpu: convert ia64 to dynamic percpu and drop the old one, take#2

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Message-ID: <87hbukxcau.wl%peter@chubb.wattle.id.au>
Date:	Thu, 01 Oct 2009 09:06:33 +1000
From:	Peter Chubb <peterc@...ato.unsw.edu.au>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <teheo@...e.de>, Nick Piggin <npiggin@...e.de>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	linux-ia64 <linux-ia64@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHSET percpu#for-next] percpu: convert ia64 to dynamic percpu and drop the old one, take#2

>>>>> "Tony" == Tony Luck <Luck> writes:

>> Tony: Could we use a global register for the per cpu address? That
>> would make IA64 work similar to sparc.

Tony> Would that be a useful trade of resources for convenience?
Tony> We've already hard-wired r13 for "current". Grabbing another one
Tony> would require fixing (since user code will have clobbered it).
Tony> Possibly re-working any existing code that already uses whatever
Tony> register you choose.

r3, r4 and r5 are currently unused by the kernel, and unused
by GCC and ICC.   Only hand-written assembler and weird compilers use
those registers(and my virtual-machine monitor :-().  If you wanted to
experiment, that'd be a starting place.  

I'm not sure of the advantage though -- TLB mapping is relatively
cheap, and we're no longer hard-wiring the translation register.

You';d have to do somne careful benchmarking on a wide variety of
workloads and machines to get a definitive answer.

---
Dr Peter Chubb        www.nicta.com.au      peter DOT chubb AT nicta.com.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia
>From Imagination to Impact                       Imagining the (ICT) Future
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