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 <pbonzini@xxxxxxxxxx> 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). > > Paolo > Unpopular opinion: Let's keep it simple first (straight rcu) and optimize later on. -- Thanks, David
