Simple drop of Phase 2 content

roycaihw · roycaihw · commit d95be9b18abb · 2025-06-17T00:08:48.000Z
diff --git a/keps/sig-node/4205-psi-metric/README.md b/keps/sig-node/4205-psi-metric/README.md
@@ -1,4 +1,4 @@
-# KEP-4205: PSI Based Node Conditions
+# KEP-4205: Expose PSI Metrics
 <!-- toc -->
 - [Release Signoff Checklist](#release-signoff-checklist)
 - [Summary](#summary)
@@ -8,22 +8,18 @@
 - [Proposal](#proposal)
   - [User Stories (Optional)](#user-stories-optional)
     - [Story 1](#story-1)
-    - [Story 2](#story-2)
   - [Risks and Mitigations](#risks-and-mitigations)
 - [Design Details](#design-details)
-    - [Phase 1](#phase-1)
-      - [CPU](#cpu)
-      - [Memory](#memory)
-      - [IO](#io)
-    - [Phase 2 to add PSI based actions.](#phase-2-to-add-psi-based-actions)
+    - [CPU](#cpu)
+    - [Memory](#memory)
+    - [IO](#io)
   - [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)
-    - [Phase 1: Alpha](#phase-1-alpha)
-    - [Phase 2: Alpha](#phase-2-alpha)
+    - [Alpha](#alpha)
     - [Beta](#beta)
     - [GA](#ga)
     - [Deprecation](#deprecation)
@@ -85,7 +81,7 @@ Items marked with (R) are required *prior to targeting to a milestone / release*
 
 ## Summary
 
-This KEP proposes adding support in kubelet to read Pressure Stall Information (PSI) metric pertaining to CPU, Memory and IO resources exposed from cAdvisor and runc. This will enable kubelet to report node conditions which will be utilized to prevent scheduling of pods on nodes experiencing significant resource constraints.
+This KEP proposes adding support in kubelet to read Pressure Stall Information (PSI) metric pertaining to CPU, Memory and IO resources exposed from cAdvisor and runc.
 
 ## Motivation
 
@@ -98,11 +94,6 @@ In short, PSI metric are like barometers that provide fair warning of impending
 This proposal aims to:
 1. Enable the kubelet to have the PSI metric of cgroupv2 exposed from cAdvisor and Runc.
 2. Enable the pod level PSI metric and expose it in the Summary API.
-3. Utilize the node level PSI metric to set node condition and node taints.
-
-It will have two phases:
-Phase 1: includes goal 1, 2 
-Phase 2: includes goal 3
 
 ### Non-Goals
 
@@ -119,23 +110,13 @@ Today, to identify disruptions caused by resource crunches, Kubernetes users nee
 install node exporter to read PSI metric. With the feature proposed in this enhancement, 
 PSI metric will be available for users in the Kubernetes metrics API.
 
-#### Story 2
-
-Kubernetes users want to prevent new pods to be scheduled on the nodes that have resource starvation. By using PSI metric, the kubelet will set Node Condition to avoid pods being scheduled on nodes under high resource pressure. The node controller could then set a [taint on the node based on these new Node Conditions](https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/#taint-nodes-by-condition).
-
 ### Risks and Mitigations
 
-There are no significant risks associated with Phase 1 implementation that involves integrating
+There are no significant risks associated with integrating
 the PSI metric in kubelet from either from cadvisor runc libcontainer library or kubelet's CRI runc libcontainer implementation which doesn't involve any shelled binary operations.
 
-Phase 2 involves utilizing the PSI metric to report node conditions. There is a potential
-risk of early reporting for nodes under pressure. We intend to address this concern
-by conducting careful experimentation with PSI threshold values to identify the optimal
-default threshold to be used for reporting the nodes under heavy resource pressure.
-
 ## Design Details
 
-#### Phase 1
 1. Add new Data structures PSIData and PSIStats corresponding to the PSI metric output format as following:
 
 ```
@@ -194,49 +175,6 @@ type NodeStats struct {
 }
 ```
 
-#### Phase 2 to add PSI based actions.
-**Note:** These actions are tentative, and will depend on different the outcome from testing and discussions with sig-node members, users, and other folks. 
-
-1. Introduce a new kubelet config parameter, pressure threshold, to let users specify the pressure percentage beyond which the kubelet would report the node condition to disallow workloads to be scheduled on it.
-
-2. Add new node conditions corresponding to high PSI (beyond threshold levels) on CPU, Memory and IO.
-
-```go
-// These are valid conditions of the node. Currently, we don't have enough information to decide
-// node condition.
-const (
-…
-	// Conditions based on pressure at system level cgroup.
-	NodeSystemCPUContentionPressure    NodeConditionType = "SystemCPUContentionPressure"
-	NodeSystemMemoryContentionPressure NodeConditionType = "SystemMemoryContentionPressure"
-	NodeSystemDiskContentionPressure   NodeConditionType = "SystemDiskContentionPressure"
-
-	// Conditions based on pressure at kubepods level cgroup.
-	NodeKubepodsCPUContentionPressure    NodeConditionType = "KubepodsCPUContentionPressure"
-	NodeKubepodsMemoryContentionPressure NodeConditionType = "KubepodsMemoryContentionPressure"
-	NodeKubepodsDiskContentionPressure   NodeConditionType = "KubepodsDiskContentionPressure"
-)
-```
-
-3. Kernel collects PSI data for 10s, 60s and 300s timeframes. To determine the optimal observation timeframe, it is necessary to conduct tests and benchmark performance. 
-In theory, 10s interval might be rapid to taint a node with NoSchedule effect. Therefore, as an initial approach, opting for a 60s timeframe for observation logic appears more appropriate. 
-
-  Add the observation logic to add node condition and taint as per following scenarios:
-  * If avg60 >= threshold, then record an event indicating high resource pressure.
-  * If avg60 >= threshold and is trending higher i.e. avg10 >= threshold, then set Node Condition for high resource contention pressure. This should ensure no new pods are scheduled on the nodes under heavy resource contention pressure.
-  * If avg60 >= threshold for a node tainted with NoSchedule effect,  and is trending lower i.e. avg10 <= threshold, record an event mentioning the resource contention pressure is trending lower.
-  * If avg60 < threshold for a node tainted with NoSchedule effect, remove the NodeCondition.
-
-4. Collaborate with sig-scheduling to modify TaintNodesByCondition feature to integrate new taints for the new Node Conditions introduced in this enhancement.
-
-* `node.kubernetes.io/memory-contention-pressure=:NoSchedule`
-* `node.kubernetes.io/cpu-contention-pressure=:NoSchedule`
-* `node.kubernetes.io/disk-contention-pressure=:NoSchedule`
-
-5. Perform experiments to finalize the default optimal pressure threshold value.
-
-6. Add a new feature gate PSINodeCondition, and guard the node condition related logic behind the feature gate. Set `--feature-gates=PSINodeCondition=true` to enable the feature.
-
 ### Test Plan
 
 <!--
@@ -318,20 +256,12 @@ We expect no non-infra related flakes in the last month as a GA graduation crite
 
 ### Graduation Criteria
 
-#### Phase 1: Alpha
+#### Alpha
 
 - PSI integrated in kubelet behind a feature flag.
 - Unit tests to check the fields are populated in the 
   Summary API response.
 
-#### Phase 2: Alpha
-
-- Implement Phase 2 of the enhancement which enables kubelet to 
-report node conditions based off PSI values.
-- Initial e2e tests completed and enabled if CRI implementation supports
-it.
-- Add documentation for the feature.
-
 #### Beta
 
 - Feature gate is enabled by default.
@@ -406,7 +336,7 @@ well as the [existing list] of feature gates.
 -->
 
 - [X] Feature gate (also fill in values in `kep.yaml`)
-  - Feature gate name: PSINodeCondition
+  - Feature gate name: KubeletPSI
   - Components depending on the feature gate: kubelet
 - [ ] Other
   - Describe the mechanism:
@@ -421,7 +351,7 @@ well as the [existing list] of feature gates.
 Any change of default behavior may be surprising to users or break existing
 automations, so be extremely careful here.
 -->
-Not in Phase 1. Phase 2 is TBD in K8s 1.31.
+No.
 
 ###### Can the feature be disabled once it has been enabled (i.e. can we roll back the enablement)?
 
@@ -438,9 +368,8 @@ NOTE: Also set `disable-supported` to `true` or `false` in `kep.yaml`.
 Yes
 
 ###### What happens if we reenable the feature if it was previously rolled back?
-When the feature is disabled, the Node Conditions will still exist on the nodes. However,
-they won't be any consumers of these node conditions. When the feature is re-enabled,
-the kubelet will override out of date Node Conditions as expected.
+No PSI metrics will be availabe in kubelet Summary API nor Prometheus metrics if the
+feature was rolled back.
 
 ###### Are there any tests for feature enablement/disablement?
 
@@ -513,13 +442,9 @@ Ideally, this should be a metric. Operations against the Kubernetes API (e.g.,
 checking if there are objects with field X set) may be a last resort. Avoid
 logs or events for this purpose.
 -->
-For Phase 1:
 Use `kubectl get --raw "/api/v1/nodes/{$nodeName}/proxy/stats/summary"` to call Summary API. If the PSIStats field is seen in the API response,
 the feature is available to be used by workloads.
 
-For Phase 2:
-TBD
-
 ###### How can someone using this feature know that it is working for their instance?
 
 <!--
@@ -658,10 +583,11 @@ NA
 ## Implementation History
 
 - 2023/09/13: Initial proposal
+- 2025/06/10: Drop Phase 2 from this KEP. Phase 2 will be tracked in its own KEP to allow separate milestone tracking
 
 ## Drawbacks
 
-No drawbacks in Phase 1 identified. There's no reason the enhancement should not be
+No drawbacks identified. There's no reason the enhancement should not be
 implemented. This enhancement now makes it possible to read PSI metric without installing
 additional dependencies
 
