Skip draining nodes that are already cordoned

[adriananeci]

We ran into a situation were we had to disable Descheduler because it was attempting to drain nodes which were cordoned, causing conflicts with cluster upgrade.

We found that Descheduler was attempting to evict every outdated Node in the cluster during the worker re-image because it reacts to a Node taint which is added by the node controller when a Node is marked as unschedulable. This caused the cluster to be disrupted at a high rate which made it very unstable.

This PR ignores nodes that are marked as unschedulable so that Descheduler won't conflict with node controller taints. Not sure why the code was in place, but commented out.

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[adriananeci]

@damemi Thanks for your thoughts. Please be aware that in case of Taint based Evictions:

Note:
Kubernetes automatically adds a toleration for node.kubernetes.io/not-ready and node.kubernetes.io/unreachable with tolerationSeconds=300, unless you, or a controller, set those tolerations explicitly.

These automatically-added tolerations mean that Pods remain bound to Nodes for 5 minutes after one of these problems is detected.

In this case Descheduler might trigger at any time in this 5 minutes (default) timeframe.

Unfortunately I don't have the logs anymore since the issue happed a while ago and they were rotated.

If agreed, I'm happy to gate this behaviour with a feature setting.

[ingvagabund]

Hello @adriananeci.

Not sure why the code was in place, but commented out.

Some strategies like LowNodeUtilization, HighNodeUtilization or RemoveDuplicates need to take into account all nodes when selecting nodes for eviction. Given the ReadyNodes (invoker of IsReady) is invoked in the main descheduling loop, we need to take into account even nodes which are getting drained/unschedulable.

Instead of ignoring nodes which are unschedulable I suggest we perform a check in the pod evictor and refuse to evict a pod when a node has certain list of known taints added (e.g. node.kubernetes.io/not-ready, node.kubernetes.io/unreachable, node.kubernetes.io/unschedulable).

[JaneLiuL]

agree with @ingvagabund , some pods have tolerations , we still need to check some strategies on all nodes.

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@adriananeci: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-descheduler-test-e2e-k8s-master-1-24	382e76ba6160d3a9be2b2c953c4bfd6026b6a819	link	true	/test pull-descheduler-test-e2e-k8s-master-1-24

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/lifecycle rotten

[ingvagabund]

/remove-lifecycle rotten

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/lifecycle stale

[ingvagabund]

/remove-lifecycle stale

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/lifecycle stale

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/lifecycle rotten

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/close

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@k8s-triage-robot: Closed this PR.

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/close

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