kvm: Iterate over only vcpus that are preempted

This helps in filtering out the eligible candidates further and
thus potentially helps in quickly allowing preempted lockholders to run.
Note that if a vcpu was spinning during preemption we filter them
by checking whether they are preempted due to pause loop exit.
Reviewed-by: NChegu Vinod <chegu_vinod@hp.com>
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NRaghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

virt/kvm/kvm_main.c

@@ -1768,6 +1768,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 				continue;
 			} else if (pass && i > last_boosted_vcpu)
 				break;
+			if (!ACCESS_ONCE(vcpu->preempted))
+				continue;
 			if (vcpu == me)
 				continue;
 			if (waitqueue_active(&vcpu->wq))