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 16:47:51 +0200 Message-ID: <4D53FA97.4090007@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> <4D53E5FB.5090109@redhat.com> <4D53EC7B.3030903@siemens.com> <4D53F5A4.8050403@redhat.com> <4D53F770.6040908@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]:32760 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751110Ab1BJOr4 (ORCPT ); Thu, 10 Feb 2011 09:47:56 -0500 In-Reply-To: <4D53F770.6040908@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On 02/10/2011 04:34 PM, Jan Kiszka wrote: > On 2011-02-10 15:26, Avi Kivity wrote: > > On 02/10/2011 03:47 PM, Jan Kiszka wrote: > >>>> > >>>> 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. > > > > What else does kvm_lock protect? > > Someone tried to write a locking.txt and stated that it's also > protecting enabling/disabling hardware virtualization. But that guy may > have overlooked something. Right. I guess splitting that lock makes sense. > > > > I think we could simply reduce the amount of time we hold kvm_lock. > > Pick a vm, ref it, list_move_tail(), unlock, then do the actual > > shrinking. Of course taking a ref must be done carefully, we might > > already be in kvm_destroy_vm() at that time. > > > > Plain mutex held across the whole mmu_shrink loop is still simpler and > should be sufficient - unless we also have to deal with scalability > issues if that handler is able to run concurrently. But based on how we > were using kvm_lock so far... I don't think a mutex would work for kvmclock_cpufreq_notifier(). At the very least, we'd need a preempt_disable() there. At the worst, the notifier won't like sleeping. -- error compiling committee.c: too many arguments to function