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

[Alexander Graf <agraf@suse.de>]

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@vger.kernel.org/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.


Alex