From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757955AbYG1Sg6 (ORCPT ); Mon, 28 Jul 2008 14:36:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753794AbYG1Sgt (ORCPT ); Mon, 28 Jul 2008 14:36:49 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:47643 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754083AbYG1Sgs (ORCPT ); Mon, 28 Jul 2008 14:36:48 -0400 Date: Mon, 28 Jul 2008 11:32:33 -0700 (PDT) From: Linus Torvalds To: Mike Travis cc: Rusty Russell , Ingo Molnar , Linux Kernel Mailing List , Andrew Morton , Al Viro Subject: Re: [git pull] cpus4096 fixes In-Reply-To: <488E06F0.4070404@sgi.com> Message-ID: References: <20080727190601.GA764@elte.hu> <200807281042.12860.rusty@rustcorp.com.au> <488E06F0.4070404@sgi.com> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 28 Jul 2008, Mike Travis wrote: > > Sorry, I didn't know that was the protocol. And yes, the clever idea of > compacting the memory is a good one (wish I would have thought of it... ;-) > But, and it's a big but, if you really have 4096 cpus present (not NR_CPUS, > but nr_cpu_ids), then 2MB is pretty much chump change. Umm. Yes, it's chump change, but if you compile a kernel to be generic, and you actually only have a few CPU's, it's no longer chump change. > But I'll redo the patch again. Here's a trivial setup, that is even tested. It's _small_ too. /* cpu_bit_bitmap[0] is empty - so we can back into it */ #define MASK_DECLARE_1(x) [x+1][0] = 1ul << (x) #define MASK_DECLARE_2(x) MASK_DECLARE_1(x), MASK_DECLARE_1(x+1) #define MASK_DECLARE_4(x) MASK_DECLARE_2(x), MASK_DECLARE_2(x+2) #define MASK_DECLARE_8(x) MASK_DECLARE_4(x), MASK_DECLARE_4(x+4) static const unsigned long cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)] = { MASK_DECLARE_8(0), MASK_DECLARE_8(8), MASK_DECLARE_8(16), MASK_DECLARE_8(24), #if BITS_PER_LONG > 32 MASK_DECLARE_8(32), MASK_DECLARE_8(40), MASK_DECLARE_8(48), MASK_DECLARE_8(56), #endif }; static inline const cpumask_t *get_cpu_mask(unsigned int nr) { const unsigned long *p = cpu_bit_bitmap[1 + nr % BITS_PER_LONG]; p -= nr / BITS_PER_LONG; return (const cpumask_t *)p; } that should be all you need to do. Honesty in advertizing: my "testing" was some trivial user-space harness, maybe I had some bug in it. But at least it's not _horribly_ wrong. And yes, this has the added optimization from Viro of overlapping the cpumask_t's internally too, rather than making them twice the size. So with 4096 CPU's, this should result 32.5kB of static const data. Linus