On Thu, 17 Aug 2017 09:04:14 +0200 Alexander Graf <agraf@xxxxxxx> 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@xxxxxxxxxxxxxxx/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.
