Include "computed QoS" instead of "status QoS" in validation error when in-place resource updates result in recalculated QoS

[jkyros]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Currently:

• In-place resource updates can result in Pod QoS changing due to recalculation, which causes the update to fail validation
• That validation failure error currently includes the existing QoS value from the Pod's status, rather than the new computed QoS

This PR:

• changes the error message to output the computed QoS, so the user can know what the QoS value was recalculated to and hopefully correlate it to their attempted resource changes

Example:

Take this pod:

apiVersion: v1
kind: Pod
metadata:
  name: sleeper
  namespace: default
spec:
  containers:
  - args:
    - -c
    - sleep infinity
    command:
    - /bin/sh
    image: registry.k8s.io/ubuntu-slim:0.1
    imagePullPolicy: IfNotPresent
    name: sleeper
    resources:
      requests:
        cpu: 25m
        memory: 100Mi

Remove the resources to make it recalculate to BestEffort:

$ kubectl patch pod sleeper --type="strategic" -p'{"spec":{"containers":[{"name":"sleeper","resources":{"$patch":"delete"}}]}}'

Observe the validation output (the pod is already Burstable, but the error says Burstable is the invalid value):

The Pod "sleeper" is invalid: 
* metadata: Invalid value: "Burstable": Pod QoS is immutable
* spec: Forbidden: pod updates may not change fields other than `spec.containers[*].image`,`spec.initContainers[*].image`,`spec.activeDeadlineSeconds`,`spec.tolerations` (only additions to existing tolerations),`spec.terminationGracePeriodSeconds` (allow it to be set to 1 if it was previously negative)

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

In-place resource updates that recalculate pod QoS now properly include the recalculated QoS value with the validation failure

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jkyros
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[jkyros]

/sig node

[bart0sh]

/triage accepted
/priority important-longterm
/ok-to-test
/cc

[bart0sh]

/retest

[iholder101]

nit

Suggested change

	computedNewQOS := qos.ComputePodQOS(newPod)
	if qos.GetPodQOS(oldPod) != computedNewQOS {
	allErrs = append(allErrs, field.Invalid(fldPath, computedNewQOS, "Pod QoS is immutable"))
	if computedNewQOS := qos.ComputePodQOS(newPod); qos.GetPodQOS(oldPod) != computedNewQOS {
	allErrs = append(allErrs, field.Invalid(fldPath, computedNewQOS, "Pod QoS is immutable"))

[iholder101]

Is it possible/easy to understand if there's an in-place scaling taking place? If it is, the problem can be made much clearer if we mention this was caused due to a resize IMO. WDYT?

[jkyros]

Yes, I think the only time this message can come out is when there is an in-place scaling taking place because:

• the function we're in ( ValidatePodUpdate) seems to only validate the spec, while the QoS is in the status
• we're comparing "old" QoS to "computed new" QoS, so "new" QoS is ignored

So I think at least here it's just kind of quietly ignoring the case where you directly change the QoS.

So hmm, keeping with the tone of the other validation errors, maybe something like pod resource changes may not change pod QoS, pod QoS is immutable ?