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Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

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On 17/08/2017 12:20, David Hildenbrand wrote:
> 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@redhat.com> 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...
> 
> Guess I am still missing the point why RCU cannot be used here.

Because the body of kvm_foreach_vcpu might sleep.

Paolo

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[PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
  [PATCH RFC 1/2] KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
    Re: [PATCH RFC 1/2] KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC Christian Borntraeger <borntraeger@de.ibm.com> - 2017-08-21 15:50 +0200
  [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
    Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 10:10 +0200
    Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 13:20 +0200
      Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 19:00 +0200
      Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 19:00 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Alexander Graf <agraf@suse.de> - 2017-08-17 09:10 +0200
    Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 09:40 +0200
      Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 11:20 +0200
        Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 11:30 +0200
          Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 11:50 +0200
            Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:00 +0200
              Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 12:20 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:30 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 12:30 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:40 +0200
    Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 17:00 +0200
      Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Alexander Graf <agraf@suse.de> - 2017-08-17 21:20 +0200
        Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-18 16:20 +0200
          Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Marc Zyngier <marc.zyngier@arm.com> - 2017-08-18 16:30 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 09:40 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 09:40 +0200

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