Kubernetes Scheduler operator

The Kubernetes Scheduler operator manages and updates the Kubernetes Scheduler deployed on top of OpenShift. The operator is based on OpenShift library-go framework and it is installed via Cluster Version Operator (CVO).

It contains the following components:

• Operator
• Bootstrap manifest renderer
• Installer based on static pods
• Configuration observer

By default, the operator exposes Prometheus metrics via metrics service. The metrics are collected from following components:

• Kubernetes Scheduler operator

Configuration

The configuration for the Kubernetes Scheduler is the result of merging:

• a default config
• an observed config (compare observed values above) from the spec schedulers.config.openshift.io.

All of these are sparse configurations, i.e. unvalidated json snippets which are merged in order to form a valid configuration at the end.

Scheduling profiles

The following profiles are currently provided:

• HighNodeUtilization
• LowNodeUtilization
• NoScoring

Each of these enables cluster-wide scheduling. Configured via Scheduler custom resource:

$ oc get scheduler cluster -o yaml

apiVersion: config.openshift.io/v1
kind: Scheduler
metadata:
  name: cluster
  spec:
    mastersSchedulable: false
    policy:
      name: ""
    profile: LowNodeUtilization
  ...

HighNodeUtilization

This profile disables NodeResourcesBalancedAllocation and NodeResourcesFit plugin with (LeastAllocated type) and enables NodeResourcesFit plugin (with MostAllocated type). Favoring nodes that have a high allocation of resources. In the past the profile corresponded to disabling NodeResourcesLeastAllocated and NodeResourcesBalancedAllocation plugins and enabling NodeResourcesMostAllocated plugin.

LowNodeUtilization

The default list of scheduling profiles as provided by the kube-scheduler.

NoScoring

This profiles disabled all scoring plugins.

Debugging

Operator also expose events that can help debugging issues. To get operator events, run following command:

$ oc get events -n  openshift-cluster-kube-scheduler-operator

This operator is configured via KubeScheduler custom resource:

$ oc describe kubescheduler

apiVersion: operator.openshift.io/v1
kind: KubeScheduler
metadata:
  name: cluster
spec:
  managementState: Managed
  ...

$ oc explain kubescheduler

to learn more about the resource itself.

The current operator status is reported using the ClusterOperator resource. To get the current status you can run follow command:

$ oc get clusteroperator/kube-scheduler

Developing and debugging the operator

In the running cluster cluster-version-operator is responsible for maintaining functioning and non-altered elements. In that case to be able to use custom operator image one has to perform one of these operations:

1. Set your operator in umanaged state, see here for details, in short:

oc patch clusterversion/version --type='merge' -p "$(cat <<- EOF
spec:
  overrides:
  - group: apps/v1
    kind: Deployment
    name: kube-scheduler-operator
    namespace: openshift-kube-scheduler-operator
    unmanaged: true
EOF
)"

2. Scale down cluster-version-operator:

oc scale --replicas=0 deploy/cluster-version-operator -n openshift-cluster-version

IMPORTANT: This apprach disables cluster-version-operator completly, whereas previous only tells it to not manage a kube-scheduler-operator!

After doing this you can now change the image of the operator to the desired one:

oc patch pod/openshift-kube-scheduler-operator-<rand_digits> -n openshift-kube-scheduler-operator -p '{"spec":{"containers":[{"name":"kube-scheduler-operator-container","image":"<user>/cluster-kube-scheduler-operator"}]}}'

Developing and debugging the bootkube bootstrap phase

The operator image version used by the installer bootstrap phase can be overridden by creating a custom origin-release image pointing to the developer's operator :latest image:

$ IMAGE_ORG=<user> make images
$ docker push <user>/origin-cluster-kube-scheduler-operator

$ cd ../cluster-kube-apiserver-operator
$ IMAGES=cluster-kube-scheduler-operator IMAGE_ORG=<user> make origin-release
$ docker push <user>/origin-release:latest

$ cd ../installer
$ OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE=docker.io/<user>/origin-release:latest bin/openshift-install cluster ...