enhancement(victim selection): improve same job-priority victim selection

[hajnalmt]

What type of PR is this?

/kind rfe
/kind feature

What this PR does / why we need it:

This PR improves same job-priority victim selection in BuildVictimsPriorityQueue by applying plugin job compare first and using TaskOrderFn as tie-break when job ordering is equal.
The behavior is applied in both the preemptor-missing path and the same-queue path so creation timestamp is not the implicit final decision maker.

Which issue(s) this PR fixes:

Fixes #5112

Special notes for your reviewer:

• Key logic change is in pkg/scheduler/framework/session_plugins.go.
• Focused unit coverage is in pkg/scheduler/framework/session_plugins_victim_order_test.go.
• The unit test is intentionally in framework_test package to avoid import cycle with real plugin wiring.
• The victim selection process was quite controversial for these tie-break scenarios as:
  return lv.CreationTimestamp.Before(&rv.CreationTimestamp) means return true when lv is older than rv, so if two jobs have the same priority the order is asceding by timestamp, for the jobOrder resulting in oldest first job ordering. For the victims we have negated this via !ssn.JobOrderFn(...) resulting in youngest first victim selection, so we evicted younger jobs on the cluster instead of the older jobs in the reclaim and preempt actions.
• AI usage disclosure: A little AI assistance was used in the creation of this PR, but the author thoroughly guided the implementation process and rewrote a good chunk of the generated parts.
• AI rhyme:
  In volcano’s queue where victims flow,
  when job ranks tie, task rules shall show;
  no timestamp shall decide the fight,
  task-order keeps the choice set right.

Does this PR introduce a user-facing change?

Improve scheduler victim selection tie-break behavior for same job-priority scenarios by using task-order when job-order ties in BuildVictimsPriorityQueue.

[volcano-sh-bot]

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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 significantly enhances the victim selection mechanism within the scheduler's BuildVictimsPriorityQueue. By introducing a dedicated JobOrderCompareFn and a new tie-breaking logic that prioritizes plugin-defined job comparison and then task ordering, it ensures a more deterministic and robust selection process. This change addresses scenarios where jobs have the same priority, preventing reliance on implicit factors like creation timestamps and leading to more predictable preemption decisions.

Highlights

• Refactored Job Ordering Logic: Introduced JobOrderCompareFn to centralize plugin-based job comparison, returning an integer for clearer ordering, and updated JobOrderFn to utilize this new comparison function.
• Enhanced Victim Selection Tie-breaking: Modified BuildVictimsPriorityQueue to use JobOrderCompareFn first, and then TaskOrderFn as a tie-breaker when job ordering is equal, ensuring a more deterministic selection.
• Consistent Tie-breaking Application: Applied the new job-then-task ordering logic consistently across both preemptor-missing and same-queue victim selection paths within BuildVictimsPriorityQueue.
• Added Unit Tests: Included new unit tests in session_plugins_victim_order_test.go to specifically cover the improved victim selection tie-break behavior.

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Changelog

• release-note
  • Improve scheduler victim selection tie-break behavior for same job-priority scenarios by using task-order when job-order ties in BuildVictimsPriorityQueue.

Activity

• The author provided special notes for reviewers, highlighting key logic changes and unit test locations.
• The author disclosed the use of AI assistance in the creation of the PR, noting thorough guidance and significant rewrites.
• The author included an AI-generated rhyme related to the PR's content.

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[gemini-code-assist]

Code Review

This pull request effectively improves victim selection for jobs with the same priority by introducing a more robust tie-breaking mechanism. The refactoring of JobOrderFn into JobOrderCompareFn is a clean approach that enhances code reuse. The new logic in BuildVictimsPriorityQueue correctly uses TaskOrderFn as a tie-breaker, avoiding reliance on creation timestamps. The addition of a dedicated unit test is also a valuable contribution. I have one minor suggestion to improve comment clarity.

[Copilot]

Pull request overview

Improves victim selection ordering when multiple victims belong to jobs with the same job priority by using plugin job ordering first and TaskOrderFn as a deterministic tie-breaker (instead of implicitly falling back to timestamp/UID behavior).

Changes:

• Introduce JobOrderCompareFn to expose plugin-only job ordering comparison results.
• Update BuildVictimsPriorityQueue to apply job ordering first and task ordering when job ordering ties (including the “preemptor job missing” path).
• Add focused unit tests for the updated victim ordering behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
pkg/scheduler/framework/session_plugins.go	Adds plugin-only job compare and uses task-order as tie-break in victim PQ ordering.
pkg/scheduler/framework/session_plugins_victim_order_test.go	Adds unit tests asserting the victim ordering outcome in both preemptor-job-present/missing paths.

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