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

[Paolo Bonzini <pbonzini@redhat.com>]

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).

Paolo