Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751473Ab3GXLFN (ORCPT ); Wed, 24 Jul 2013 07:05:13 -0400 Received: from smtp.citrix.com ([66.165.176.89]:37660 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751215Ab3GXLFL (ORCPT ); Wed, 24 Jul 2013 07:05:11 -0400 X-IronPort-AV: E=Sophos;i="4.89,734,1367971200"; d="scan'208";a="38946239" Date: Wed, 24 Jul 2013 12:05:05 +0100 From: Stefano Stabellini X-X-Sender: sstabellini@kaball.uk.xensource.com To: Konrad Rzeszutek Wilk CC: Ian Campbell , Stefano Stabellini , , , , Subject: Re: [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages In-Reply-To: <20130723190521.GA7073@phenom.dumpdata.com> Message-ID: References: <1374600466-3229-1-git-send-email-stefano.stabellini@eu.citrix.com> <1374602409.6623.138.camel@hastur.hellion.org.uk> <20130723190521.GA7073@phenom.dumpdata.com> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) 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 Content-Length: 1994 Lines: 44 On Tue, 23 Jul 2013, Konrad Rzeszutek Wilk wrote: > On Tue, Jul 23, 2013 at 07:00:09PM +0100, Ian Campbell wrote: > > On Tue, 2013-07-23 at 18:27 +0100, Stefano Stabellini wrote: > > > +static int __cpuinit balloon_cpu_notify(struct notifier_block *self, > > > + unsigned long action, void *hcpu) > > > +{ > > > + int cpu = (long)hcpu; > > > + switch (action) { > > > + case CPU_UP_PREPARE: > > > + if (per_cpu(balloon_scratch_page, cpu) != NULL) > > > + break; > > > > Thinking about this a bit more -- do we know what happens to the per-cpu > > area for a CPU which is unplugged and then reintroduced? Is it preserved > > or is it reset? > > > > If it is reset then this gets more complicated :-( We might be able to > > use the core mm page reference count, so that when the last reference is > > removed the page is automatically reclaimed. We can obviously take a > > reference whenever we add a mapping of the trade page, but I'm not sure > > we are always on the path which removes such mappings... Even then you > > could waste pages for some potentially large amount of time each time > > you replug a VCPU. > > > > Urg, I really hope the per-cpu area is preserved! > > It is. During bootup time you see this: > > [ 0.000000] smpboot: Allowing 128 CPUs, 96 hotplug CPU > [ 0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:128 nr_node_ids:1 > > which means that all of the per_CPU are shrunk down to 128 (from > CONFIG_NR_CPUS=512 was built with) and stays for the lifetime of the kernel. > > You might have to clear it when the vCPU comes back up though - otherwise you > will have garbage. I don't see anything in the hotplug code that would modify the value of the per_cpu area of offline cpus. -- 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/