From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU Date: Thu, 10 Feb 2011 15:19:55 +0200 Message-ID: <4D53E5FB.5090109@redhat.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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , kvm , Zachary Amsden To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1135 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752114Ab1BJNUA (ORCPT ); Thu, 10 Feb 2011 08:20:00 -0500 In-Reply-To: <4D53E4C0.5000605@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: 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. > And I don't see the need for call_rcu in the > vm deletion path. synchronize_rcu() is fine for vm destruction. -- error compiling committee.c: too many arguments to function