From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cornelia Huck Subject: Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Date: Thu, 17 Aug 2017 09:36:12 +0200 Message-ID: <20170817093612.024cc4bc.cohuck@redhat.com> References: <20170816194037.9460-1-rkrcmar@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Cc: Radim =?UTF-8?B?S3LEjW3DocWZ?= , 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 , Paolo Bonzini To: Alexander Graf Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thu, 17 Aug 2017 09:04:14 +0200 Alexander Graf wrote: > On 16.08.17 21:40, Radim Krčmář wrote: > > The goal is to increase KVM_MAX_VCPUS without worrying about memory > > impact of many small guests. > > > > This is a second out of three major "dynamic" options: > > 1) size vcpu array at VM creation time > > 2) resize vcpu array when new VCPUs are created > > 3) use a lockless list/tree for VCPUs > > > > The disadvantage of (1) is its requirement on userspace changes and > > limited flexibility because userspace must provide the maximal count on > > start. The main advantage is that kvm->vcpus will work like it does > > now. It has been posted as "[PATCH 0/4] KVM: add KVM_CREATE_VM2 to > > allow dynamic kvm->vcpus array", > > http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1377285.html > > > > The main problem of (2), this series, is that we cannot extend the array > > in place and therefore require some kind of protection when moving it. > > RCU seems best, but it makes the code slower and harder to deal with. > > The main advantage is that we do not need userspace changes. > > Creating/Destroying vcpus is not something I consider a fast path, so > why should we optimize for it? The case that needs to be fast is execution. > > 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.