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 <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). >> >> 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
