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

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Date:	Wed, 30 Sep 2009 19:49:50 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Peter Chubb <peterc@...ato.unsw.edu.au>
cc:	"Luck, Tony" <tony.luck@...el.com>, 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

On Thu, 1 Oct 2009, Peter Chubb wrote:

> 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.

Dynamic and static per cpu variables could use relative access to that
register. This would reduce code size, avoid the use of a TLB entry.

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

I have some patches here that make heavy use of dynamic percpu allocations
in the allocators to optimize the alloc / free paths.

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