support stateful scheduler plugins

[pohly]

What type of PR is this?

/kind feature

What this PR does / why we need it:

The assumption so far was that plugins which are active during cluster autoscaling can do their work exclusively based on the information maintained by cluster autoscaler about nodes and pods on those nodes. This assumption doesn't hold for plugins which need to observe and potentially modify other cluster state, like the dynamic resource allocation plugin.

Such plugins need a way to do their own snapshotting of the cluster state at the start of a simulation and then update that snapshot while the cluster autoscaler simulates the placement of pods on nodes.

The new ClusterAutoScalerPlugin interface provides a mechanism for this. It needs to be called in a few places if a plugin implements that interface.

Which issue(s) this PR fixes:

Related-to: kubernetes/kubernetes#118612

Special notes for your reviewer:

If the dynamic resource allocation plugin is inactive, then no plugin implements the new interface and these code changes become mostly no-ops.

The corresponding PR in k/k needs to be merged first.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [KEP]: https://github.com/kubernetes/enhancements/issues/3063

[k8s-ci-robot]

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/lifecycle stale

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[pohly]

/close

For DRA with structured parameters, I am using the approach instead that each pod scheduling cycle reconstructs state from caches. This might be worse from a performance perspective, but is easier to integrate with autoscaling. We can come back to this approach here if (and only if) needed.

[k8s-ci-robot]

@pohly: Closed this PR.

In response to this:

/close

For DRA with structured parameters, I am using the approach instead that each pod scheduling cycle reconstructs state from caches. This might be worse from a performance perspective, but is easier to integrate with autoscaling. We can come back to this approach here if (and only if) needed.

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