Re: [PATCHv3] kvm/irqchip: Speed up KVM_SET_GSI_ROUTING

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 17/01/14 15:03, Paolo Bonzini wrote:
> Il 16/01/2014 21:22, Michael S. Tsirkin ha scritto:
>>> [PATCHv3] kvm/irqchip: Speed up KVM_SET_GSI_ROUTING
>>>
>>> When starting lots of dataplane devices the bootup takes very long
>>> on my s390 system(prototype irqfd code). With larger setups we are even
>>> able to trigger some timeouts in some components.
>>> Turns out that the KVM_SET_GSI_ROUTING ioctl takes very
>>> long (strace claims up to 0.1 sec) when having multiple CPUs.
>>> This is caused by the  synchronize_rcu and the HZ=100 of s390.
>>>
>>> Lets use the expedited variant to speed things up as suggested by
>>> Michael S. Tsirkin
>>>
>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>> ---
>>>  virt/kvm/irqchip.c |    2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
>>> index 20dc9e4..dbcfde7 100644
>>> --- a/virt/kvm/irqchip.c
>>> +++ b/virt/kvm/irqchip.c
>>> @@ -226,7 +226,7 @@ int kvm_set_irq_routing(struct kvm *kvm,
>>>  	kvm_irq_routing_update(kvm, new);
>>>  	mutex_unlock(&kvm->irq_lock);
>>>  
>>> -	synchronize_rcu();
>>> +	synchronize_rcu_expedited();
>>>  
>>>  	new = old;
>>>  	r = 0;
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> 
> Well... I love to contradict myself, so: no way this can be accepted
> this close to the end of the merge window.  :(
> 
> synchronize_rcu_expedited() forces a context switch on all CPUs, even
> those that are not running KVM.  Thus, this patch might help a guest DoS
> its host by changing the IRQ routing tables in a loop.
> 
> So this will have to wait for 3.15.  We have ~2 months to do performance
> measurements on the v2 patch.  Sorry.

Any chance that you or Michael can give some performance feedback on v2? All
my lab systems are s390 and not x86...