From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Travis Subject: Re: [patch 02/41] cpu alloc: The allocator Date: Wed, 04 Jun 2008 08:30:23 -0700 Message-ID: <4846B50F.5060808@sgi.com> References: <20080530035620.587204923@sgi.com> <483F8AF7.9000309@cosmosbay.com> <200805301552.25423.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from netops-testserver-3-out.sgi.com ([192.48.171.28]:60880 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750959AbYFDPa0 (ORCPT ); Wed, 4 Jun 2008 11:30:26 -0400 In-Reply-To: <200805301552.25423.rusty@rustcorp.com.au> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Rusty Russell Cc: Christoph Lameter , Eric Dumazet , akpm@linux-foundation.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, David Miller , Peter Zijlstra Rusty Russell wrote: > On Friday 30 May 2008 15:20:45 Christoph Lameter wrote: >> On Fri, 30 May 2008, Eric Dumazet wrote: >>>> +static DEFINE_PER_CPU(UNIT_TYPE, area[UNITS]); >>> area[] is not guaranteed to be aligned on anything but 4 bytes. >>> >>> If someone then needs to call cpu_alloc(8, GFP_KERNEL, 8), it might get >>> an non aligned result. >>> >>> Either you should add an __attribute__((__aligned__(PAGE_SIZE))), >>> or take into account the real address of area[] in cpu_alloc() to avoid >>> waste of up to PAGE_SIZE bytes >>> per cpu. >> I think cacheline aligning should be sufficient. People should not >> allocate large page aligned objects here. > > I vaguely recall there were issues with this in the module code. They might > be gone now, but failing to meet alignment contraints without a big warning > would suck. > > But modifying your code to consider the actual alignment is actually pretty > trivial, AFAICT. > > Cheers, > Rusty. So paraphrasing my earlier email, we should add: bitmap_find_free_area(bitmap, nbits, size, align, alignbase) so that > cacheline alignment is possible? My thinking is that if we do go to true dynamically sized cpu_alloc area then allocating PAGE_SIZE units may be both practical and worthwhile...? Thanks, Mike