Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760205AbYGJPaf (ORCPT ); Thu, 10 Jul 2008 11:30:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759481AbYGJP2P (ORCPT ); Thu, 10 Jul 2008 11:28:15 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:39059 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759476AbYGJP2N (ORCPT ); Thu, 10 Jul 2008 11:28:13 -0400 Message-ID: <48762A3B.5050104@linux-foundation.org> Date: Thu, 10 Jul 2008 10:26:51 -0500 From: Christoph Lameter User-Agent: Thunderbird 2.0.0.14 (Windows/20080421) MIME-Version: 1.0 To: "H. Peter Anvin" 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> In-Reply-To: <48761C06.3020003@zytor.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1398 Lines: 31 H. Peter Anvin wrote: > Noone has talked about the actual placement of the percpu segment data. But the placement of the percpu segment data is a problem because of the way we currently have the linker calculate offsets. I have had kernel configurations where I changed the placement of the percpu segment leading to linker failures because the percpu segment was not in 2G range of the code segment! This is a particular problem if we have a large number of processors (like 4096) that each require a sizable segment of virtual address space up there for the per cpu allocator. > None of this affects the absolute positioning of the data. The final > address are determined by: > > fs_base + rip + offset > or > fs_base + offset > > ... respectively. fs_base is an arbitrary 64-bit number; rip (in the > kernel) is in the range [-2 GB + CONFIG_PHYSICAL_START, 0), and offset > is in the range [-2 GB, 2 GB). Well the zero based results in this becoming always gs_base + absolute address in per cpu segment Why are RIP based references cheaper? The offset to the per cpu segment is certainly more than what can be fit into 16 bits. -- 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/