From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU Date: Thu, 10 Feb 2011 14:47:39 +0100 Message-ID: <4D53EC7B.3030903@siemens.com> References: <4D512EF7.8040409@siemens.com> <4D512F3B.1080107@siemens.com> <4D53BB02.20206@redhat.com> <4D53CCAB.8040204@siemens.com> <4D53DB54.90605@redhat.com> <4D53DDD3.5020704@siemens.com> <4D53E063.1040004@redhat.com> <4D53E0A9.2030501@redhat.com> <4D53E4C0.5000605@siemens.com> <4D53E5FB.5090109@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , kvm , Zachary Amsden To: Avi Kivity Return-path: Received: from goliath.siemens.de ([192.35.17.28]:24898 "EHLO goliath.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750766Ab1BJNry (ORCPT ); Thu, 10 Feb 2011 08:47:54 -0500 In-Reply-To: <4D53E5FB.5090109@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2011-02-10 14:19, Avi Kivity wrote: > On 02/10/2011 03:14 PM, Jan Kiszka wrote: >> On 2011-02-10 13:57, Avi Kivity wrote: >>> On 02/10/2011 02:56 PM, Avi Kivity wrote: >>>>> What's the benefit? The downside is a bit more complexity as you need an >>>>> additional callback handler. >>>> >>>> >>>> synchronize_rcu() can be very slow (its a systemwide operation), and >>>> mmu_shrink() can be called often on a loaded system. >>>> >>> >>> In fact this just shows that vm_list is not a good candidate for rcu; >>> rcu is useful where most operations are reads, but if we discount stats, >>> most operations on vm_list are going to be writes. >> >> Accept for mmu_shrink, which is write but not delete, thus works without >> that slow synchronize_rcu. > > I don't really see how you can implement list_move_rcu(), it has to be > atomic or other users will see a partial vm_list. Right, even if we synchronized that step cleanly, rcu-protected users could miss the moving vm during concurrent list walks. What about using a separate mutex for protecting vm_list instead? Unless I missed some detail, mmu_shrink should allow blocking. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux