Binding preemption Design for Karmada scheduler

[seanlaii]

What type of PR is this?
/kind documentation
/kind feature

What this PR does / why we need it:
This PR adds a design proposal for binding-level preemption in the Karmada scheduler.
Currently, priority-based scheduling only orders the queue — a high-priority binding that arrives after cluster resources are consumed by lower-priority bindings remains pending indefinitely. This proposal introduces preemption so the scheduler can evict lower-priority bindings to make room, scoped to single-cluster Divided scheduling in Phase 1 with estimator-based node-level simulation planned for Phase 2.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a comprehensive design proposal for binding-level preemption within the Karmada scheduler. The core purpose is to resolve resource contention by allowing higher-priority workloads to preempt lower-priority ones, preventing high-priority tasks from being indefinitely blocked. The proposal outlines a strategic, phased approach, beginning with a foundational summary-based preemption for single-cluster environments and planning for a more advanced estimator-based system in the future. This enhancement significantly improves resource utilization and fairness for critical workloads.

Highlights

• Binding-Level Preemption Introduced: Introduced binding-level preemption to the Karmada scheduler, enabling high-priority ResourceBindings to evict lower-priority ones when cluster resources are insufficient, initially scoped to single-cluster scenarios.
• Phased Implementation Strategy: Designed a two-phase implementation, starting with summary-based preemption (Phase 1) and evolving to estimator-based node-level simulation for more precise victim selection (Phase 2).
• API Extension for Preemption Policy: Added a PreemptionPolicy field to ResourceBindingSpec.SchedulePriority, which defaults to Never and requires explicit opt-in for PreemptLowerPriority.
• In-Memory Preemption Claims Mechanism: Implemented an in-memory preemption claims store to coordinate preemption, prevent repeated preemption attempts, and reserve cluster capacity for the preemptor.
• Integration with Existing Karmada Features: Reused the existing SchedulePriority and PriorityClass mechanisms for preemption ordering and integrated the eviction process with GracefulEviction.
• Feature Gating: Gated the new preemption functionality behind the PriorityBasedPreemptiveScheduling alpha feature gate, ensuring it is opt-in and can be safely rolled out.

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive proposal for binding-level preemption within the Karmada scheduler, specifically for single-cluster scheduling. The feature aims to allow high-priority ResourceBindings to evict lower-priority ones when cluster resources are insufficient. The proposal details a two-phase implementation, starting with summary-based preemption and progressing to estimator-based preemption for more precise victim selection. A review comment suggests enhancing the clarity of eviction messages for victim bindings by including the workload's kind, namespace, and name for better user understanding.

[gemini-code-assist]

The eviction message for the victim binding could be more informative. Currently, it uses preemptor.Namespace and preemptor.Name, which refer to the ResourceBinding's metadata. This might be confusing for users, as the ResourceBinding name is often generated (e.g., my-app-deployment) and doesn't directly reflect the workload's identity.

To improve clarity, I suggest using the workload's kind, namespace, and name from preemptor.Spec.Resource. This would generate a more user-friendly message like Preempted by Deployment default/my-app (priority=100).

This change should also be reflected in the Observability section's event table for the Preempted event.

Suggested change

	workv1alpha2.WithMessage(fmt.Sprintf("Preempted by %s/%s (priority=%d)",
	preemptor.Namespace, preemptor.Name,
	preemptor.Spec.SchedulePriority.Priority)),
	workv1alpha2.WithMessage(fmt.Sprintf("Preempted by %s %s/%s (priority=%d)",
	preemptor.Spec.Resource.Kind, preemptor.Spec.Resource.Namespace, preemptor.Spec.Resource.Name,
	preemptor.Spec.SchedulePriority.Priority)),

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.03%. Comparing base (5ba1066) to head (16f153a).
⚠️ Report is 2 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7327   +/-   ##
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  Coverage   42.02%   42.03%           
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  Files         874      874           
  Lines       53551    53551           
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+ Hits        22505    22509    +4     
+ Misses      29354    29351    -3     
+ Partials     1692     1691    -1     

Flag	Coverage Δ
unittests	42.03% <ø> (+<0.01%)	⬆️

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