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Preemption admission check controller. KEP update. #1178

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Preemption admission check controller. KEP update. #1178

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Preemption admission check controller. KEP update.
trasc Oct 4, 2023
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Review remarks.
trasc Oct 5, 2023
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trasc Oct 6, 2023
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Check the pending workloads in order if their priority.
trasc Oct 19, 2023
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37 changes: 36 additions & 1 deletion keps/993-two-phase-admission/README.md
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Expand Up @@ -12,6 +12,7 @@
- [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
- [Risks and Mitigations](#risks-and-mitigations)
- [Design Details](#design-details)
- [Preemption Admission Check Controller](#preemption-admission-check-controller)
- [Test Plan](#test-plan)
- [Prerequisite testing updates](#prerequisite-testing-updates)
- [Unit Tests](#unit-tests)
Expand Down Expand Up @@ -206,7 +207,7 @@ pass AND there are some AdmissionChecks configured, AND the
workload is not in on-hold retry state from some check, it will:

1. Fill the Admission field in workload, with the desired flavor assignment.
2. Not do any preemptions yet (unless BookCapacity is set to true).
2. Not do any preemptions yet (unless the check uses `Anytime` preemption policy).
3. Set "QuotaReserved" to true.

Kueue will only pass as many pods into "QuotaReserved" as there would
Expand Down Expand Up @@ -253,6 +254,40 @@ The controller implementing a particular check should:
* After approving the workload, keep an eye on the check if it starts failing,
fail the check and cause the workload to move back to the suspended state.

### Preemption Admission Check Controller
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In this proposal, the time to evict the preemption candidates varies based on the preemptor state
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the scheduler will not issue the eviction during it's process instead it will set a `Pending` admission check
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that is manged by a new built-in admission check controller.

The **Preemption Admission Check Controller** will:

- Watch for a change in state of the workloads pending preemption.
- Watch for workloads that are finishing execution and therefore releasing quota.
- Watch for AdmissionCheck changes, since the preemption policy can change.

The preemption controller uses the kueue cache, since it needs to check the state of workloads admitted to the ClusterQueues.

At every run the controller will get the list of workloads pending preemption.

The workloads pending preemption are divided into:
- `preemtingLetaer` - Workloads having at least one check that uses AfterCheckPassedOrOnDemand policy with the state `pending`.
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- `preemtingNow` - Workloads that expect to be able to issue evictions or potentially change their preemption state in the current cycle.

Then:
1. Remove all workloads from the snapshot.
2. For every workload in `preemtingNow` , in the order of their priority:
- If it can fit without the need of evicting other workloads
- Set its admission check to `Ready`
- Add it to the snapshot
- If it cannot fit
- Get an updated list of eviction candidates
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Please clarify which ones are candidates? Is it only workloads that are fully Admitted? I don't think you want to consider ones that only have ResourceQuota.

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The candidates are picked from the list of workloads that hold a quota reservation, just because maybe a admission check of a workload is transitioning slowly to ready dose not mean that it should be able to hold its quota against a higher priority workload.

- If the updated list is not empty.
- Issue the eviction to the candidates.
- Add it to the snapshot
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- If the updated list is empty, meaning the preemption cannot be done.
- Set its admission check to `Ready`
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### Test Plan

[ x ] I/we understand the owners of the involved components may require updates to
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