Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752702AbdHQJTb (ORCPT ); Thu, 17 Aug 2017 05:19:31 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:33982 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752199AbdHQJRF (ORCPT ); Thu, 17 Aug 2017 05:17:05 -0400 Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array To: Cornelia Huck , Alexander Graf Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , linux-mips@linux-mips.org, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Marc Zyngier , Christian Borntraeger , James Hogan , Christoffer Dall , Paul Mackerras , David Hildenbrand References: <20170816194037.9460-1-rkrcmar@redhat.com> <20170817093612.024cc4bc.cohuck@redhat.com> From: Paolo Bonzini Message-ID: Date: Thu, 17 Aug 2017 11:16:59 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170817093612.024cc4bc.cohuck@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 831 Lines: 18 On 17/08/2017 09:36, Cornelia Huck wrote: >> What if we just sent a "vcpu move" request to all vcpus with the new >> pointer after it moved? That way the vcpu thread itself would be >> responsible for the migration to the new memory region. Only if all >> vcpus successfully moved, keep rolling (and allow foreign get_vcpu again). >> >> That way we should be basically lock-less and scale well. For additional >> icing, feel free to increase the vcpu array x2 every time it grows to >> not run into the slow path too often. > > I'd prefer the rcu approach: This is a mechanism already understood > well, no need to come up with a new one that will likely have its own > share of problems. What Alex is proposing _is_ RCU, except with a homegrown synchronize_rcu. Using kvm->srcu seems to be the best of both worlds. Paolo