Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to call_rcu in vhost_set_memory() in vhost kernel module?

[Paolo Bonzini]

Il 12/05/2014 11:28, Gonglei (Arei) ha scritto:
> From previous discussion:
> https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg04925.html
> we know that you are going to replace RCU in KVM_SET_GSI_ROUTING with SRCU. 
> Though
> SRCU is quite better than originally RCU, in our test case this cannot satisfy 
> our needs. Our VMs
> work in telecom scenario, VMs report CPU and memory usage to balance node each 
> second, and
> balance node dispatch works to different VMs according to VM load. Since this 
> balance needs
> high accuracy, IRQ affinity settings in VM also need high accuracy, so we 
> balance IRQ affinity in
> every 0.5s. So for telecom scenario, KVM_SET_GSI_ROUTING IOCTL needs much 
> optimization.
> And in live migration case, VHOST_SET_MEM_TABLE needs attention.
>
> We tried to change synchronize_rcu() to call_rcu() with rate limit, but rate 
> limit is not easy to
> configure. Do you have better ideas to achieve this? Thanks.

Perhaps we can check for cases where only the address is changing, and 
poke at an existing struct kvm_kernel_irq_routing_entry without doing 
any RCU synchronization?

As long as kvm_set_msi_irq only reads address_lo once, it should work.

VHOST_SET_MEM_TABLE is a different problem.  What happens in userspace 
that leads to calling that ioctl?  Can we remove it altogether, or delay 
it to after the destination has started running?

Paolo