Re: [RFC] kvm: x86: export vCPU halted state to sysfs

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 01, 2018 at 09:15:15PM +0100, Radim Krčmář wrote:
> 2018-02-01 12:54-0500, Luiz Capitulino:
> > 
> > Libvirt needs to know when a vCPU is halted. To get this information,
> 
> I don't see why upper level management should care about that, a single
> bit about halted state that can be incorrect at the time it is processed
> seems of very limited use.
> 
> (A much more sensible data point would be the fraction of time when VCPU
>  was running or runnable, which is roughly what you get by sampling the
>  halted state.)
> 
> A halted vCPU it might even be halted in guest mode, so KVM doesn't know
> about that state (unless you force a VM exit), which would complicate
> the collection a bit more ... but really, what is the data being used
> for?

I'm unclear what usage is being made of the halted state info, as it was an
addition to libvirt from a 3rd party. My guess is that it was used to
see if the vCPU had been brought online by the guest OS at all, after being
hotplugged. As you say, the info is pretty useless once the vCPU is actually
online & in use, becuase it changes so quickly.

> 
> User might care about the state, for obscure reasons, but that isn't a
> performance problem.
> 
> > libvirt has started using the query-cpus command from QEMU. However,
> > if in kernel irqchip is in use, query-cpus will force all vCPUs
> > to user-space since they have to issue the KVM_GET_MP_STATE ioctl.
> 
> Libvirt knows if KVM exits to userspace on halts, so it can just query
> QEMU in that case and in the other case, there is a very dirty
> "solution" that works on all architectures right now:
> 
>   grep kvm_vcpu_block /proc/$vcpu_task/stack
> 
> If you get something, the vcpu is halted in KVM.
> 
> > This has catastrophic implications to low-latency workloads like
> > KVM-RT and zero packet loss with DPDK. To make matters worse, there's
> > an OpenStack service called ceilometer that causes libvirt to
> > issue query-cpus every few minutes.
> 
> I'd expect that people running these workloads can setup the system. :(
> 
> I bet that ceilometer just mindlessly collects everything, so we should
> be able to configure libvirt to collect only some stats.  Either libvirt
> or upper layer would decide what is too expensive for its usefulness.
> 
> > The solution proposed in this patch is to export the vCPU
> > halted state in the already existing vcpu directory in sysfs.
> > This way, libvirt can read the vCPU halted state from sysfs and avoid
> > using the query-cpus command. This solution seems to be sufficient
> > for libvirt needs, but it has the following cons:
> > 
> >  * vcpu information in sysfs lives in a debug directory, so
> >    libvirt would be basing its API on debug info
> 
> (It pains me to say there probably already are tools that depend on
>  kvm/debug.)
> 
> It's slightly better than the stack hack, but needs more code in kernel
> and the interface is in a gray compatibility zone, so I'd like to know
> why does userspace do that in the first place.
> 
> >  * Currently, only x86 supports the vcpu dir in sysfs, so
> >    we'd have to expand this to other archs (should be doable)
> > 
> > If we agree that this solution is feasible, I'll work on extending
> > the vcpu debug information to other archs for my next posting.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > @@ -6273,6 +6273,7 @@ void kvm_arch_exit(void)
> >  
> >  int kvm_vcpu_halt(struct kvm_vcpu *vcpu)
> >  {
> > +	kvm_vcpu_set_halted(vcpu);
> 
> There is no point to care about !lapic_in_kernel().  I'd move the logic
> into vcpu_block() to be shared among all architectures.
> 
> >  	++vcpu->stat.halt_exits;
> >  	if (lapic_in_kernel(vcpu)) {
> >  		vcpu->arch.mp_state = KVM_MP_STATE_HALTED;

Regards,
Daniel
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