Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752542AbdHQKUJ (ORCPT ); Thu, 17 Aug 2017 06:20:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33534 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752362AbdHQKUH (ORCPT ); Thu, 17 Aug 2017 06:20:07 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 8C12178ED0 Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=david@redhat.com Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array To: Paolo Bonzini , Cornelia Huck Cc: Alexander Graf , =?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 References: <20170816194037.9460-1-rkrcmar@redhat.com> <20170817093612.024cc4bc.cohuck@redhat.com> <20170817112829.7795820a.cohuck@redhat.com> <69d47131-2405-b2be-60d7-e40fbd17592a@redhat.com> <6609138c-6a29-5106-39a8-3219ce47f01a@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <6b2816ca-d8c0-0819-efdb-c30fd3d1a0e9@redhat.com> Date: Thu, 17 Aug 2017 12:20:02 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Thu, 17 Aug 2017 10:20:06 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1946 Lines: 50 On 17.08.2017 12:18, Paolo Bonzini wrote: > On 17/08/2017 11:55, David Hildenbrand wrote: >> On 17.08.2017 11:44, Paolo Bonzini wrote: >>> On 17/08/2017 11:28, Cornelia Huck wrote: >>>> On Thu, 17 Aug 2017 11:16:59 +0200 >>>> Paolo Bonzini wrote: >>>> >>>>> 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. >>>> >>>> I'm worried a bit about the 'homegrown' part, though. >>> >>> I agree, that's why I'm suggesting SRCU instead. But it's a trick that >>> has its uses. For example, if you were only doing reads from a work >>> queue, flush_work_queue could be used as the "homegrown >>> synchronize_rcu". In KVM you might use kvm_make_all_cpus_request, I guess. >>> >>>> I also may be misunderstanding what Alex means with "vcpu move"... >>> >>> My interpretation was "resizing the array" (so it moves in memory). >> >> Unpopular opinion: Let's keep it simple first (straight rcu) and >> optimize later on. > > RCU vs. SRCU is about correctness, not optimization... > > Paolo > Guess I am still missing the point why RCU cannot be used here. -- Thanks, David