Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758881AbYGJQlS (ORCPT ); Thu, 10 Jul 2008 12:41:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753138AbYGJQlH (ORCPT ); Thu, 10 Jul 2008 12:41:07 -0400 Received: from terminus.zytor.com ([198.137.202.10]:33159 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751639AbYGJQlF (ORCPT ); Thu, 10 Jul 2008 12:41:05 -0400 Message-ID: <487639ED.7000502@zytor.com> Date: Thu, 10 Jul 2008 12:33:49 -0400 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: Christoph Lameter CC: Jeremy Fitzhardinge , "Eric W. Biederman" , Ingo Molnar , Mike Travis , Andrew Morton , Jack Steiner , linux-kernel@vger.kernel.org, Arjan van de Ven Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses References: <20080709165129.292635000@polaris-admin.engr.sgi.com> <20080709200757.GD14009@elte.hu> <48751B57.8030605@goop.org> <48751CF9.4020901@linux-foundation.org> <4875209D.8010603@goop.org> <48752CCD.30507@linux-foundation.org> <48753C99.5050408@goop.org> <487555A8.2050007@zytor.com> <487556A5.5090907@goop.org> <4876194E.4080205@linux-foundation.org> <48761C06.3020003@zytor.com> <48762A3B.5050104@linux-foundation.org> <48762DD2.5090802@zytor.com> <487637A1.4080403@linux-foundation.org> In-Reply-To: <487637A1.4080403@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1640 Lines: 36 Christoph Lameter wrote: > H. Peter Anvin wrote: > >> but there is a distinct lack of wiggle room, which can be resolved >> either by using negative offsets, or by moving the kernel text area up a >> bit from -2 GB. > > Lets say we reserve 256MB of cpu alloc space per processor. > > On a system with 4k processors this will result in the need for 1TB virtual address space for per cpu areas (note that there may be more processors in the future). Preferably we would calculate the address of the per cpu area by > > PERCPU_START_ADDRESS + PERCPU_SIZE * smp_processor_id() > > instead of looking it up in a table because that will save a memory access on per_cpu(). It will, but it might still be a net loss due to higher load on the TLB (you're effectively using the TLB to do the table lookup for you.) On the other hand, Mike points out that once we move away from fixed-sized segments we pretty much have to use virtual addresses anyway(*). > The first percpu area would ideally be the per cpu segment generated by the linker. > > How would that fit into the address map? In particular the 2G distance between code and the first per cpu area must not be violated unless we go to a zero based approach. If with "zero-based" you mean "nonzero gs_base for the boot CPU" then yes, you're right. Note again that that is completely orthogonal to RIP-based versus absolute. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/