It reduces the number of kinds of cluster events the scheduler needs to subscribe/handle. ([#121571](https://github.com/kubernetes/kubernetes/pull/121571), [@sanposhiho](https://github.com/sanposhiho))

	// so far and save the cost of a deep copy.
	if apiequality.Semantic.DeepEqual(newPod.Spec, oldPod.Spec) {
		return allErrs
	}

	if qos.GetPodQOS(oldPod) != qos.ComputePodQOS(newPod) {
		allErrs = append(allErrs, field.Invalid(fldPath, newPod.Status.QOSClass, "Pod QoS is immutable"))
	}