From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Travis Subject: Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled Date: Mon, 08 Dec 2008 06:29:33 -0800 Message-ID: <493D2F4D.3090702@sgi.com> References: <200812072125.45757.rusty@rustcorp.com.au> <200812081635.35166.rusty@rustcorp.com.au> <493CED04.6020209@redhat.com> <200812082220.42790.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , kvm-devel , linux-kernel@vger.kernel.org To: Rusty Russell Return-path: Received: from relay2.sgi.com ([192.48.179.30]:60643 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751945AbYLHO31 (ORCPT ); Mon, 8 Dec 2008 09:29:27 -0500 In-Reply-To: <200812082220.42790.rusty@rustcorp.com.au> Sender: kvm-owner@vger.kernel.org List-ID: Rusty Russell wrote: > On Monday 08 December 2008 20:16:44 Avi Kivity wrote: >> Rusty Russell wrote: >>>> This isn't on stack, so it isn't buying us anything. >>>> >>> It's the CONFIG_NR_CPUS=4096 but nr_cpu_ids=4 case which we win using >>> dynamic allocation. Gotta love distribution kernels. >>> >>> >> What does it buy? 4096/8 = 512 bytes statically allocated? > > It adds up, and 4096 seems to be only the start of the insanityH^H^Hfun. The real win though is when cpumask_size represents the actual size of the cpumask (based on # of possible cpus) instead of the pre-configured size of NR_CPUS. So for 99.9% of the systems (having 64 or fewer cpus), the savings will be 504 bytes not allocated. > >>> Not quite. If !CONFIG_CPUMASK_OFFSTACK, cpumask_var_t == cpumask_t[1]. >>> Blame Linus :) >>> >> Hm, is there a C trick which will error out when allocating something on >> the stack, but work when allocating statically? I can think of >> something to do the reverse, but that doesn't help. > > We also need to prevent assignment, eg: > > *foo = *bar; > > Because when we allocate them, we'll cut them to size. > > Cheers, > Rusty.