Re: [PATCH 4/5] KVM: cpu hotplug support

[Avi Kivity <avi@qumranet.com>]

Ingo Molnar wrote:
> * Andrew Morton <akpm@osdl.org> wrote:
>
>   
> > On Tue, 30 Jan 2007 14:56:16 -0000
> > Avi Kivity <avi@qumranet.com> wrote:
> >
> >     
> > > +static void decache_vcpus_on_cpu(int cpu)
> > > +{
> > > +	struct kvm *vm;
> > > +	struct kvm_vcpu *vcpu;
> > > +	int i;
> > > +
> > > +	spin_lock(&kvm_lock);
> > > +	list_for_each_entry(vm, &vm_list, vm_list)
> > > +		for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> > > +			vcpu = &vm->vcpus[i];
> > > +			/*
> > > +			 * If the vcpu is locked, then it is running on some
> > > +			 * other cpu and therefore it is not cached on the
> > > +			 * cpu in question.
> > > +			 *
> > > +			 * If it's not locked, check the last cpu it executed
> > > +			 * on.
> > > +			 */
> > > +			if (mutex_trylock(&vcpu->mutex)) {
> > > +				if (vcpu->cpu == cpu) {
> > > +					kvm_arch_ops->vcpu_decache(vcpu);
> > > +					vcpu->cpu = -1;
> > > +				}
> > > +				mutex_unlock(&vcpu->mutex);
> > > +			}
> > > +		}
> > > +	spin_unlock(&kvm_lock);
> > > +}
> > >       
> > The trylock is unpleasing.  Perhaps kvm_lock should be a mutex or 
> > something?
> >     
>
> this is a special case. The vcpu->mutex acts as a 'this vcpu is running 
> right now' flag as well - hence the trylock signals: is it running right 
> now or not - if it's not running we do not have to 'decache' it. But i 
> agree and i already suggested to Avi to change kvm_lock to be a mutex - 
> but this wont change the trylock.
>   

To elaborate a little: replacing mutex_trylock() with mutex_lock() will 
cause unbounded latency as we wait for the vcpu to be descheduled.  In 
this case, we're only interested in descheduled vcpus, so there's no 
need to wait.

kvm is a bit funny in how it likes to pin cpus.

--
error compiling committee.c: too many arguments to function

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