Pod Scheduling Readiness

Pods were considered ready for scheduling once created. Kubernetes scheduler does its due diligence to find nodes to place all pending Pods. However, in a real-world case, some Pods may stay in a "miss-essential-resources" state for a long period. These Pods actually churn the scheduler (and downstream integrators like Cluster AutoScaler) in an unnecessary manner.

By specifying/removing a Pod's .spec.schedulingGates, you can control when a Pod is ready to be considered for scheduling.

Configuring Pod schedulingGates

The schedulingGates field contains a list of strings, and each string literal is perceived as a criteria that Pod should be satisfied before considered schedulable. This field can be initialized only when a Pod is created (either by the client, or mutated during admission). After creation, each schedulingGate can be removed in arbitrary order, but addition of a new scheduling gate is disallowed.

Usage example

To mark a Pod not-ready for scheduling, you can create it with one or more scheduling gates like this:

pods/pod-with-scheduling-gates.yaml

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  schedulingGates:
  - name: example.com/foo
  - name: example.com/bar
  containers:
  - name: pause
    image: registry.k8s.io/pause:3.6

After the Pod's creation, you can check its state using:

kubectl get pod test-pod

The output reveals it's in SchedulingGated state:

NAME       READY   STATUS            RESTARTS   AGE
test-pod   0/1     SchedulingGated   0          7s

You can also check its schedulingGates field by running:

kubectl get pod test-pod -o jsonpath='{.spec.schedulingGates}'

The output is:

[{"name":"example.com/foo"},{"name":"example.com/bar"}]

To inform scheduler this Pod is ready for scheduling, you can remove its schedulingGates entirely by re-applying a modified manifest:

pods/pod-without-scheduling-gates.yaml

apiVersion: v1
kind: Pod
metadata:
  name: test-pod
spec:
  containers:
  - name: pause
    image: registry.k8s.io/pause:3.6

You can check if the schedulingGates is cleared by running:

kubectl get pod test-pod -o jsonpath='{.spec.schedulingGates}'

The output is expected to be empty. And you can check its latest status by running:

kubectl get pod test-pod -o wide

Given the test-pod doesn't request any CPU/memory resources, it's expected that this Pod's state get transited from previous SchedulingGated to Running:

NAME       READY   STATUS    RESTARTS   AGE   IP         NODE  
test-pod   1/1     Running   0          15s   10.0.0.4   node-2

Observability

The metric scheduler_pending_pods comes with a new label "gated" to distinguish whether a Pod has been tried scheduling but claimed as unschedulable, or explicitly marked as not ready for scheduling. You can use scheduler_pending_pods{queue="gated"} to check the metric result.

Mutable Pod Scheduling Directives

You can mutate scheduling directives of Pods while they have scheduling gates, with certain constraints. At a high level, you can only tighten the scheduling directives of a Pod. In other words, the updated directives would cause the Pods to only be able to be scheduled on a subset of the nodes that it would previously match. More concretely, the rules for updating a Pod's scheduling directives are as follows:

1. For .spec.nodeSelector, only additions are allowed. If absent, it will be allowed to be set.

2. For spec.affinity.nodeAffinity, if nil, then setting anything is allowed.

3. If NodeSelectorTerms was empty, it will be allowed to be set. If not empty, then only additions of NodeSelectorRequirements to matchExpressions or fieldExpressions are allowed, and no changes to existing matchExpressions and fieldExpressions will be allowed. This is because the terms in .requiredDuringSchedulingIgnoredDuringExecution.NodeSelectorTerms, are ORed while the expressions in nodeSelectorTerms[].matchExpressions and nodeSelectorTerms[].fieldExpressions are ANDed.

4. For .preferredDuringSchedulingIgnoredDuringExecution, all updates are allowed. This is because preferred terms are not authoritative, and so policy controllers don't validate those terms.

What's next

• Read the PodSchedulingReadiness KEP for more details