Pod Scheduling Readiness [page]deterministic

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.

 — Figure. Pod SchedulingGates

## Usage example

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

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

```bash kubectl get pod test-pod ```

The output reveals it's in `SchedulingGated` state:

```none NAME READY STATUS RESTARTS AGE test-pod 0/1 SchedulingGated 0 7s ```

You can also check its `schedulingGates` field by running:

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

The output is:

```none [{"name":"example.com/foo"},{"name":"example.com/bar"}] ```

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

You can check if the `schedulingGates` is cleared by running:

```bash 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:

```bash 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`:

```none 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 t …(trimmed)