Improve cluster-autoscaler integration

[sbueringer]

While investigating an autoscaler issue we identified a few areas of improvement of the autoscaler - CAPI integration

We should look into the following areas:

• [x] First temporary/stopgap solution for CAPI controller and autoscaler fighting over replicas during MD rollouts (see: https://github.com/kubernetes/autoscaler/issues/8494)
• [ ] Improve behavior during Machine deletion (including Node drain etc.)
  • Today autoscaler cordons/taints/drains Nodes before triggering Machine deletion
  • This means that the CAPI Machine deletion logic is not respected (pre-drain hooks, MachineDrainRules, drain observability, ...)
  • An idea: Maybe we want to disable cordon/taint/drain in autoscaler, options:
    • Via a global flag to allow disabling drain
    • Extend the CloudProvider interface with a new Method to allow disabling drain per node group
    • Extending the GetOptions method of the NodeGroup interface to allow disabling drain per node group
• [ ] Double-check that autoscaler does not scale up to many Machines based on pending Pods
  • Not entirely sure, but it looks like we observed autoscaler scaling up twice within 12 seconds because of just 1 pending Pod
• [ ] Find a final solution for autoscaling during MD rollouts
  • [ ] Improve how autoscaler triggers Machine deletion (delete-machine annotation on MS-level + MD scale down is a weak/no API)

Note: The following seems to be available today:

• Disable cordon with: --cordon-node-before-terminating=false (but disabling cordon without disabling drain seems bad)
• Disable DeletionCandidateOfClusterAutoscaler:PreferNoSchedule taint with: --max-bulk-soft-taint-count=0
  • ToBeDeletedByClusterAutoscaler:NoSchedule taint cannot be disabled today, needs a new flag (like one of the two above) (Hack: overwrite maxConcurrentNodesTainting const to 0?)

Past Slack thread: https://kubernetes.slack.com/archives/C8TSNPY4T/p1756465597770899

[sbueringer]

cc @elmiko @fabriziopandini

[sbueringer]

cc @wjunott

[elmiko]

thanks @sbueringer !

[fabriziopandini]

First point is in progress (@elmiko PR). Second point is actionable, but it requires work in the autoscaler - including checking of our proposed solution makes sense for the autoscaler maintainers. /help

Note: The long term solution need further discussion

[k8s-ci-robot]

@fabriziopandini: This request has been marked as needing help from a contributor.

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In response to this:

First point is in progress (@elmiko PR). Second point is actionable, but it requires work in the autoscaler - including checking of our proposed solution makes sense for the autoscaler maintainers. /help

Note: The long term solution need further discussion

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