Update autoscaling from zero enhancement proposal with support for platform-aware autoscale from zero

This commit updates the contract between the cluster-autoscaler Cluster API provider and the infrastructure provider's controllers that reconcile the Infrastructure Machine Template to support platform-aware autoscale from 0 in clusters consisting of nodes heterogeneous in CPU architecture and OS.

With this commit, the infrastructure providers implementing controllers to reconcile the status of their Infrastructure Machine Templates for supporting autoscale from 0 will be able to fill the status.nodeInfo stanza with additional information about the nodes.

The status.nodeInfo stanza has type corev1.NodeSystemInfo to reflect the same content, the rendered nodes' objects would store in their status field.

The cluster-autoscaler can use that information to build the node template labels `kubernetes.io/arch` and `kubernetes.io/os` if that information is present.

Suppose the pending pods that trigger the cluster autoscaler have a node selector or a requiredDuringSchedulingIgnoredDuringExecution node affinity concerning the architecture or operating system of the node where they can execute. In that case, the autoscaler will be able to filter the nodes groups options according to the architecture or operating system requested by the pod.

The users could already provide this information to the cluster autoscaler through the labels capacity annotation. However, there is no similar capability to support future labels/taints through information set by the reconcilers of the status of Infrastructure Machine Templates.

Signed-off-by: aleskandro <aleskandro@redhat.com>

Author: aleskandro

docs/book/src/developer/providers/contracts/infra-machine.md

@@ -501,7 +501,7 @@ If implementing the pause behavior, providers SHOULD surface the paused status o
 
 ### InfraMachineTemplate: support cluster autoscaling from zero
 
-As described in the enhancement [Opt-in Autoscaling from Zero][Opt-in Autoscaling from Zero], providers may implement a capacity field in machine templates to inform the cluster autoscaler about the resources available on that machine type.
+As described in the enhancement [Opt-in Autoscaling from Zero][Opt-in Autoscaling from Zero], providers may implement the `capacity` and `nodeInfo` fields in machine templates to inform the cluster autoscaler about the resources available on that machine type, the architecture, and the operating system it runs.
 
 Building on the `FooMachineTemplate` example from above, this shows the addition of a status and capacity field:
 
@@ -524,19 +524,59 @@ type FooMachineTemplateStatus struct {
 	// https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20210310-opt-in-autoscaling-from-zero.md
 	// +optional
 	Capacity corev1.ResourceList `json:"capacity,omitempty"`
+	// +optional
+	NodeInfo NodeInfo `json:"nodeInfo,omitempty,omitzero"`
+}
+
+// Architecture represents the CPU architecture of the node.
+// Its underlying type is a string and its value can be any of amd64, arm64, s390x, ppc64le.
+// +kubebuilder:validation:Enum=amd64;arm64;s390x;ppc64le
+// +enum
+type Architecture string
+
+// Example architecture constants defined for better readability and maintainability.
+const (
+    ArchitectureAmd64 Architecture = "amd64"
+    ArchitectureArm64 Architecture = "arm64"
+    ArchitectureS390x Architecture = "s390x"
+    ArchitecturePpc64le Architecture = "ppc64le"
+)
+
+// NodeInfo contains information about the node's architecture and operating system.
+// +kubebuilder:validation:MinProperties=1
+type NodeInfo struct {
+    // architecture is the CPU architecture of the node. 
+    // Its underlying type is a string and its value can be any of amd64, arm64, s390x, ppc64le.
+    // +optional
+    Architecture Architecture `json:"architecture,omitempty"`
+    // operatingSystem is a string representing the operating system of the node.
+    // This may be a string like 'linux' or 'windows'.
+    // +optional
+    OperatingSystem string `json:"operatingSystem,omitempty"`
 }
+
 ```
 
-When rendered to a manifest, the machine template status capacity field representing an instance with 500 megabytes of RAM, 1 CPU core, and 1 NVidia GPU would look like this:
+When rendered to a manifest, the machine template status capacity field representing an amd64 linux instance with 500 megabytes of RAM, 1 CPU core, and 1 NVidia GPU should look like this:
 
 ```
 status:
   capacity:
     memory: 500mb
     cpu: "1"
     nvidia.com/gpu: "1"
+   nodeInfo:
+    architecture: amd64
+    operatingSystem: linux
 ```
 
+If the information in the `nodeInfo` field is not available, the result of the autoscaling from zero operation will depend
+on the cluster autoscaler implementation. For example, the Cluster API implementation of the Kubernetes Cluster Autoscaler 
+will assume the host is running either the architecture set in the `CAPI_SCALE_ZERO_DEFAULT_ARCH` environment variable of
+the cluster autoscaler pod environment, or the amd64 architecture and Linux operating system as default values.
+
+See [autoscaling](../../../tasks/automated-machine-management/autoscaling.md).
+
 ## Typical InfraMachine reconciliation workflow
 
 A machine infrastructure provider must respond to changes to its InfraMachine resources. This process is

docs/proposals/20210310-opt-in-autoscaling-from-zero.md

@@ -107,8 +107,8 @@ node group. But, during a scale from zero situation (ie when a node group has ze
 autoscaler needs to acquire this information from the infrastructure provider.
 
 An optional status field is proposed on the Infrastructure Machine Template which will be populated
-by infrastructure providers to contain the CPU, memory, and GPU capacities for machines described by that
-template. The cluster autoscaler will then utilize this information by reading the appropriate
+by infrastructure providers to contain the CPU, CPU architecture, memory, and GPU capacities for machines
+described by that template. The cluster autoscaler will then utilize this information by reading the appropriate
 infrastructure reference from the resource it is scaling (MachineSet or MachineDeployment).
 
 A user may override the field in the associated infrastructure template by applying annotations to the
@@ -160,6 +160,13 @@ the template.  Internally, this field will be represented by a Go `map` type uti
 for the keys and `k8s.io/apimachinery/pkg/api/resource.Quantity` as the values (similar to how resource
 limits and requests are handled for pods).
 
+Additionally, the status field should contain information about the node, such as the architecture and 
+operating system. This information is not required for the autoscaler to function, but it can be useful in 
+scenarios where the autoscaler needs to make decisions for clusters with heterogeneous node groups in architecture, OS, or both.
+
+This information must be represented as a field with name `nodeInfo`, a struct with two optional subfields, 
+`architecture` and `operatingSystem`. Allowed values for architecture are `amd64`, `arm64`, `s390x`, `ppc64le`.
+
 It is worth mentioning that the Infrastructure Machine Templates are not usually reconciled by themselves.
 Each infrastructure provider will be responsible for determining the best implementation for adding the
 status field based on the information available on their platform.
@@ -175,6 +182,9 @@ const (
 // DockerMachineTemplateStatus defines the observed state of a DockerMachineTemplate
 type DockerMachineTemplateStatus struct {
     Capacity corev1.ResourceList `json:"capacity,omitempty"`
+    
+    // +optional
+    NodeInfo NodeInfo `json:"nodeInfo,omitempty,omitzero"`
 }
 
 // DockerMachineTemplate is the Schema for the dockermachinetemplates API.
@@ -188,6 +198,39 @@ type DockerMachineTemplate struct {
 ```
 _Note: the `ResourceList` and `ResourceName` referenced are from k8s.io/api/core/v1`_
 
+`NodeInfo` is a struct that contains the architecture and operating system information of the node, to implement 
+in the providers integration code.
+Its definition should look like the following:
+
+```go
+// Architecture represents the CPU architecture of the node.
+// Its underlying type is a string and its value can be any of amd64, arm64, s390x, ppc64le.
+// +kubebuilder:validation:Enum=amd64;arm64;s390x;ppc64le
+// +enum
+type Architecture string
+
+// Example architecture constants defined for better readability and maintainability.
+const (
+    ArchitectureAmd64 Architecture = "amd64"
+    ArchitectureArm64 Architecture = "arm64"
+    ArchitectureS390x Architecture = "s390x"
+    ArchitecturePpc64le Architecture = "ppc64le"
+)
+    
+// NodeInfo contains information about the node's architecture and operating system. 
+// +kubebuilder:validation:MinProperties=1
+type NodeInfo struct {
+    // architecture is the CPU architecture of the node. 
+    // Its underlying type is a string and its value can be any of amd64, arm64, s390x, ppc64le.
+    // +optional
+    Architecture Architecture `json:"architecture,omitempty"`
+    // operatingSystem is a string representing the operating system of the node.
+    // This may be a string like 'linux' or 'windows'.
+    // +optional
+    OperatingSystem string `json:"operatingSystem,omitempty"`
+}
+```
+
 When used as a manifest, it would look like this:
 
 ```
@@ -204,8 +247,13 @@ status:
     memory: 500mb
     cpu: "1"
     nvidia.com/gpu: "1"
+  nodeInfo:
+    architecture: arm64
+    operatingSystem: linux
 ```
 
+The information stored in the `status.nodeInfo` field will be used by the cluster autoscaler's scheduler simulator to determine the simulated node's labels `kubernetes.io/arch` and `kubernetes.io/os`. This logic will be implemented in the cluster autoscaler's ClusterAPI cloud provider code.
+
 #### MachineSet and MachineDeployment Annotations
 
 In cases where a user needs to provide specific resource information for a
@@ -246,6 +294,28 @@ metadata:
     capacity.cluster-autoscaler.kubernetes.io/taints: "key1=value1:NoSchedule,key2=value2:NoExecute"
 ```
 
+If the `capacity.cluster-autoscaler.kubernetes.io/labels` annotation specifies a label that would otherwise be
+generated from the fields in the `status` field of the Machine Template, the autoscaler will prioritize and use 
+the label defined in the annotation. This means any label set by the annotation will override the corresponding
+value provided by the infrastructure provider in the Machine Template status.
+
+For example, assume the following objects
+
+```yaml
+kind: MachineDeployment
+metadata:
+  annotations:
+    capacity.cluster-autoscaler.kubernetes.io/labels: kubernetes.io/arch=amd64
+---
+kind: ExampleMachineTemplate
+status:
+  nodeInfo:
+    architecture: arm64
+```
+
+The cluster autoscaler will prefer the annotation on the MachineDeployment and will predict nodes that have a 
+`kubernetes.io/arch: amd64` label on them.
+
 ### Security Model
 
 This feature will require the service account associated with the cluster autoscaler to have
@@ -318,6 +388,7 @@ office hours meeting:
 
 ## Implementation History
 
+- [X] 05/08/2025: Updated proposal to enable architecture- and OS- aware auto-scale from 0
 - [X] 09/12/2024: Added section on Implementation Status
 - [X] 01/31/2023: Updated proposal to include annotation changes
 - [X] 06/10/2021: Proposed idea in an issue or [community meeting]