Add KEP #257: Leader Only SubGroup

keps/257-Subgroup-leader-only/README.md

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+# KEP-257: SubGroup Leader Only
+
+<!--
+This is the title of your KEP. Keep it short, simple, and descriptive. A good
+title can help communicate what the KEP is and should be considered as part of
+any review.
+-->
+
+<!--
+A table of contents is helpful for quickly jumping to sections of a KEP and for
+highlighting any additional information provided beyond the standard KEP
+template.
+
+Ensure the TOC is wrapped with
+  <code>&lt;!-- toc --&rt;&lt;!-- /toc --&rt;</code>
+tags, and then generate with `hack/update-toc.sh`.
+-->
+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [User Stories (Optional)](#user-stories-optional)
+    - [Story 1](#story-1)
+  - [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
+  - [Risks and Mitigations](#risks-and-mitigations)
+- [Design Details](#design-details)
+  - [Subgroup Creation](#subgroup-creation)
+  - [TPU Environment Injection](#tpu-environment-injection)
+  - [Test Plan](#test-plan)
+      - [Prerequisite testing updates](#prerequisite-testing-updates)
+      - [Unit tests](#unit-tests)
+      - [Integration tests](#integration-tests)
+      - [e2e tests](#e2e-tests)
+  - [Graduation Criteria](#graduation-criteria)
+- [Implementation History](#implementation-history)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+<!-- /toc -->
+
+## Summary
+
+<!--
+This section is incredibly important for producing high-quality, user-focused
+documentation such as release notes or a development roadmap. It should be
+possible to collect this information before implementation begins, in order to
+avoid requiring implementors to split their attention between writing release
+notes and implementing the feature itself. KEP editors and SIG Docs
+should help to ensure that the tone and content of the `Summary` section is
+useful for a wide audience.
+
+A good summary is probably at least a paragraph in length.
+
+Both in this section and below, follow the guidelines of the [documentation
+style guide]. In particular, wrap lines to a reasonable length, to make it
+easier for reviewers to cite specific portions, and to minimize diff churn on
+updates.
+
+[documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md
+-->
+This KEP aims to extend the SubGroup feature by adding an API field that defines the type of Subgroup that will be created. The existing version
+will become "LeaderWorker", and a new type will be added to create a SubGroup that only includes the leader.
+
+
+## Motivation
+
+<!--
+This section is for explicitly listing the motivation, goals, and non-goals of
+this KEP.  Describe why the change is important and the benefits to users. The
+motivation section can optionally provide links to [experience reports] to
+demonstrate the interest in a KEP within the wider Kubernetes community.
+
+[experience reports]: https://github.com/golang/go/wiki/ExperienceReports
+-->
+
+There are cases where the resource requirements for the leader are different from the ones by the worker, while still being able to guarantee exclusive placement
+on the workers. Having a leaderOnly subgroup means that the placement of the leader will be independent of the workers, and by adding the `subgroup-exclusive-topology`, the 
+workers can be guaranteed to be scheduled on the same topology.
+
+### Goals
+
+<!--
+List the specific goals of the KEP. What is it trying to achieve? How will we
+know that this has succeeded?
+-->
+
+Add an option to create a LeaderOnly subgroup, while still being able to create subgroups on the other workers. Therefore, it should be possible to do the following split:
+(leader), (worker-1, worker-2), (worker-3, worker-4). If the desired effect is to have one subgroup for the leader, and one subgroup for the worker, subgroupSize should be set 
+to the number of workers (so size - 1). 
+
+### Non-Goals
+This KEP assumes that the leader will not request TPU resources, and thus, subGroupSize will always be assumed to be an odd number. Moreover, no TPU environment variables will be 
+injected on the leader.
+
+<!--
+What is out of scope for this KEP? Listing non-goals helps to focus discussion
+and make progress.
+-->
+
+## Proposal
+
+<!--
+This is where we get down to the specifics of what the proposal actually is.
+This should have enough detail that reviewers can understand exactly what
+you're proposing, but should not include things like API designs or
+implementation. What is the desired outcome and how do we measure success?.
+The "Design Details" section below is for the real
+nitty-gritty.
+-->
+
+### User Stories (Optional)
+
+<!--
+Detail the things that people will be able to do if this KEP is implemented.
+Include as much detail as possible so that people can understand the "how" of
+the system. The goal here is to make this feel real for users without getting
+bogged down.
+-->
+
+#### Story 1
+As a user, I should be able to create a subGroup that only includes the leader in order to be able to schedule the leader on a different topology while 
+still keeping exclusive topology on the workers.
+
+### Notes/Constraints/Caveats (Optional)
+As mentioned in Non-Goals, this KEP assumes that the leader does not request accelerator resources.
+
+<!--
+What are the caveats to the proposal?
+What are some important details that didn't come across above?
+Go in to as much detail as necessary here.
+This might be a good place to talk about core concepts and how they relate.
+-->
+
+### Risks and Mitigations
+
+<!--
+What are the risks of this proposal, and how do we mitigate? Think broadly.
+For example, consider both security and how this will impact the larger
+Kubernetes ecosystem.
+
+How will security be reviewed, and by whom?
+
+How will UX be reviewed, and by whom?
+
+Consider including folks who also work outside the SIG or subproject.
+-->
+
+## Design Details
+
+<!--
+This section should contain enough information that the specifics of your
+change are understandable. This may include API specs (though not always
+required) or even code snippets. If there's any ambiguity about HOW your
+proposal will be implemented, this is the place to discuss them.
+-->
+
+A new API field will be added under the SubGroupPolicy type. This API field will determine what type 
+of subgroup is desired. The default is LeaderWorker, which is the existing implementation of subgroup. `SubGroupSize`
+will continue to be a required field, while `Type` will be optional. 
+
+```golang
+type SubGroupPolicy struct {
+
+	// +kubebuilder:validation:Enum={LeaderWorker,LeaderAlone}
+	// +kubebuilder:default=LeaderWorker
+	Type SubGroupPolicyType `json:"subGroupPolicyType,omitempty"`
+
+    SubGroupSize *int32 `json:"subGroupSize,omitempty"`
+}
+
+type SubGroupPolicyType string
+
+const (
+	SubGroupPolicyLeaderWorker SubGroupPolicyType = "LeaderWorker"
+
+	SubGroupPolicyLeaderAlone SubGroupPolicyType = "LeaderAlone"
+)
+```
+
+A new annotation will be created to determine whether or not the subgroup type is LeaderAlone
+
+* `leaderworkerset.sigs.k8s.io/subgroup-policy-type`
+
+This annotation will only be injected in the leader pod.
+
+In order to keep backwards compatability, it will only be added if the type is `LeaderAlone`.
+
+### Subgroup Creation 
+Implementation wise, the only change needed is to not add the SubGroup labels on the leader if the SubGroupType is LeaderAlone. Effectively, this means 
+that the leader is not part of a subgroup at all. The only point of a pod being in a subgroup is to guarantee exclusive placement with the other pods 
+in the subgroup. However, since this is a one pod subgroup, there is no use case for injecting the subgroup labels on the leader. 
+
+
+```golang
+func (p *PodWebhook) Default() {
+  if foundSubGroupSize && pod.Labels[leaderworkerset.SubGroupIndexLabelKey] == "" && !leaderOnlySubGroup {
+    pod.Labels[leaderworkerset.SubGroupIndexLabelKey] = "0"
+    subGroupUniqueKey := genGroupUniqueKey(pod.Name, "0")
+    pod.Labels[leaderworkerset.SubGroupUniqueHashLabelKey] = subGroupUniqueKey
+    if subEpKey, foundSubEpKey := pod.Annotations[leaderworkerset.SubGroupExclusiveKeyAnnotationKey]; foundSubEpKey {
+      SetExclusiveAffinities(pod, subGroupUniqueKey, subEpKey, leaderworkerset.SubGroupUniqueHashLabelKey)
+    }
+  }
+}
+```
+
+### TPU Environment Injection
+When injecting TPU environment variables, we will keep the same logic that exists for when the leader doesn't request TPU resources.
+
+### Test Plan
+
+<!--
+**Note:** *Not required until targeted at a release.*
+The goal is to ensure that we don't accept enhancements with inadequate testing.
+
+All code is expected to have adequate tests (eventually with coverage
+expectations). Please adhere to the [Kubernetes testing guidelines][testing-guidelines]
+when drafting this test plan.
+
+[testing-guidelines]: https://git.k8s.io/community/contributors/devel/sig-testing/testing.md
+-->
+
+[X] I/we understand the owners of the involved components may require updates to
+existing tests to make this code solid enough prior to committing the changes necessary
+to implement this enhancement.
+
+##### Prerequisite testing updates
+
+<!--
+Based on reviewers feedback describe what additional tests need to be added prior
+implementing this enhancement to ensure the enhancements have also solid foundations.
+-->
+
+##### Unit tests
+
+
+<!--
+In principle every added code should have complete unit test coverage, so providing
+the exact set of tests will not bring additional value.
+However, if complete unit test coverage is not possible, explain the reason of it
+together with explanation why this is acceptable.
+-->
+
+<!--
+Additionally, for Alpha try to enumerate the core package you will be touching
+to implement this enhancement and provide the current unit coverage for those
+in the form of:
+- <package>: <date> - <current test coverage>
+The data can be easily read from:
+https://testgrid.k8s.io/sig-testing-canaries#ci-kubernetes-coverage-unit
+
+This can inform certain test coverage improvements that we want to do before
+extending the production code to implement this enhancement.
+-->
+
+##### Integration tests
+
+<!--
+Integration tests are contained in k8s.io/kubernetes/test/integration.
+Integration tests allow control of the configuration parameters used to start the binaries under test.
+This is different from e2e tests which do not allow configuration of parameters.
+Doing this allows testing non-default options and multiple different and potentially conflicting command line options.
+-->
+
+<!--
+This question should be filled when targeting a release.
+For Alpha, describe what tests will be added to ensure proper quality of the enhancement.
+
+For Beta and GA, add links to added tests together with links to k8s-triage for those tests:
+https://storage.googleapis.com/k8s-triage/index.html
+-->
+
+- <test>: <link to test coverage>
+
+##### e2e tests
+
+<!--
+This question should be filled when targeting a release.
+For Alpha, describe what tests will be added to ensure proper quality of the enhancement.
+
+For Beta and GA, add links to added tests together with links to k8s-triage for those tests:
+https://storage.googleapis.com/k8s-triage/index.html
+
+We expect no non-infra related flakes in the last month as a GA graduation criteria.
+-->
+
+- <test>: <link to test coverage>
+
+### Graduation Criteria
+
+<!--
+
+Clearly define what it means for the feature to be implemented and
+considered stable.
+
+If the feature you are introducing has high complexity, consider adding graduation
+milestones with these graduation criteria:
+- [Maturity levels (`alpha`, `beta`, `stable`)][maturity-levels]
+- [Feature gate][feature gate] lifecycle
+- [Deprecation policy][deprecation-policy]
+
+[feature gate]: https://git.k8s.io/community/contributors/devel/sig-architecture/feature-gates.md
+[maturity-levels]: https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#alpha-beta-and-stable-versions
+[deprecation-policy]: https://kubernetes.io/docs/reference/using-api/deprecation-policy/
+-->
+
+## Implementation History
+
+<!--
+Major milestones in the lifecycle of a KEP should be tracked in this section.
+Major milestones might include:
+- the `Summary` and `Motivation` sections being merged, signaling SIG acceptance
+- the `Proposal` section being merged, signaling agreement on a proposed design
+- the date implementation started
+- the first Kubernetes release where an initial version of the KEP was available
+- the version of Kubernetes where the KEP graduated to general availability
+- when the KEP was retired or superseded
+-->
+
+## Drawbacks
+
+<!--
+Why should this KEP _not_ be implemented?
+-->
+
+## Alternatives
+Original implementation was to have the leader have the subGroupIndex be 0, and shift all other subGroup indices are moved one to the right.
+
+```golang
+if leaderOnly {
+    return fmt.Sprint(((workerIndex - 1) / subGroupSize) + 1)
+}
+```
+
+This implementation also requires to shift the subGroupIndex back to the left when injecting TPU environment variables 
+
+```golang
+// Take worker 1 as an example with subGroupSize set to 2. It will now have subGroupIndex set to 1, which means that the start will be 3, and end will be 4. 
+// Need to shift it down by 1 for the calculation to still work out. 
+if leaderOnlyType && subGroupIndex != 0 {
+    subGroupIndex -= 1
+}
+start := subGroupSize*subGroupIndex + 1 
+end := subGroupSize * (subGroupIndex + 1)
+```
+
+Since there is no use case for having the leaderPod on its own subgroup, this implementation was scratched as it adds unnecessary complexity
+
+
+<!--
+What other approaches did you consider, and why did you rule them out? These do
+not need to be as detailed as the proposal, but should include enough
+information to express the idea and why it was not acceptable.
+-->

keps/257-Subgroup-leader-only/kep.yaml

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+title: Controller Revision
+kep-number: 238
+authors:
+  - edwinhr716
+status: implementable
+creation-date: 2025-02-27
+reviewers:
+  - kerthcet
+  - ahg-g
+approvers:
+  - kerthcet
+  - ahg-g
+
+# The most recent milestone for which work toward delivery of this KEP has been
+# done. This can be the current (upcoming) milestone, if it is being actively
+# worked on.
+latest-milestone: "v0.6.0"