feat: add scaleupTimeout

Author: sanposhiho

keps/sig-scheduling/3990-pod-topology-spread-fallback-mode/README.md

@@ -215,6 +215,7 @@ know that this has succeeded?
 - A new field `fallbackCriteria` is introduced to `PodSpec.TopologySpreadConstraint[*]` 
   - `ScaleUpFailed` to fallback when the cluster autoscaler fails to create new Node for Pod.
   - `PreemptionFailed` to fallback when preemption doesn't help make Pod schedulable.
+- A new config field `scaleupTimeout` is introduced to the PodTopologySpread plugin's configuration.
 - introduce `TriggeredScaleUp` in Pod condition 
   - change the cluster autoscaler to set it `false` when it cannot create new Node for the Pod, `true` when success.
 
@@ -273,6 +274,26 @@ topologySpreadConstraints:
 
 #### Story 2
 
+Similar to Story 1, but additionally you want to fallback when the cluster autoscaler doesn't react within the certain time.
+In that case, maybe the cluster autoscaler is down, or it takes too long time to handle pods.
+
+In this case, in addition to use `ScaleUpFailed` in `fallbackCriteria` like Story 1,
+the cluster admin can use `scaleupTimeout` in the scheduler configuration.
+
+```yaml
+apiVersion: kubescheduler.config.k8s.io/v1
+kind: KubeSchedulerConfiguration
+profiles:
+  - schedulerName: default-scheduler
+    pluginConfig:
+      - name: PodTopologySpread
+        args:
+          # trigger the fallback if a pending pod has been unschedulable for 5 min, but the cluster autoscaler hasn't yet react
+          scaleupTimeout: 5m 
+```
+
+#### Story 3
+
 Your cluster doesn't have the cluster autoscaler 
 and has some low-priority Pods to make space (often called overprovisional Pods, balloon Pods, etc.).
 Basically, you want to leverage preemption to achieve the best distribution as much as possible, 
@@ -379,6 +400,60 @@ which creates new Node for Pod typically by the cluster autoscaler.
 3. The cluster autoscaler adds `TriggeredScaleUp: false`. 
 4. The scheduler notices `TriggeredScaleUp: false` on Pod and schedules that Pod while falling back to `ScheduleAnyway` on Pod Topology Spread.
 
+### [Beta] `scaleupTimeout` in the scheduler configuration
+
+_This is targetting beta._
+
+We'll implement `ScaleupTimeout` to address the additional fallback cases,
+for example, when the cluster autoscaler is down, or the cluster autoscaler takes longer time than usual.
+
+```go
+type PodTopologySpreadArgs struct {
+  // ScaleupTimeout defines the time that the scheduler waits for the cluster autoscaler to create nodes for pending pods rejected by Pod Topology Spread.
+  // If the cluster autoscaler hasn't put any value on `TriggeredScaleUp` condition for this period of time, 
+  // the plugin triggers the fallback for topology spread constraints with `ScaleUpFailed` in `FallbackCriteria`.
+  // This is for the use cases like needing the fallback when the cluster autoscaler is down or taking too long time to react.
+  // Note that we don't guarantee that `ScaleupTimeout` means the pods are going to be retried exactly after this timeout period.
+  // The scheduler will surely retry those pods, but there might be some delay, depending on other pending pods, those pods' backoff time, and the scheduling queue's processing timing. 
+  // 
+  // This is optional; If it's empty, `ScaleUpFailed` in `FallbackCriteria` is only handled when the cluster autoscaler puts `TriggeredScaleUp: false`.
+  ScaleupTimeout *metav1.Duration
+}
+```
+
+One difficulty here is: how we move pods rejected by the PodTopologySpread plugin to activeQ/backoffQ when the timeout is reached and the fallback should be triggered.
+Currently, all the requeueing is triggered by a cluster event and we don't have any capability to trigger it by time since it's put in the unschedulable pod pool.
+
+We'll need to implement a new special cluster event, `Resource: Time`.
+The PodTopologySpread plugin (or other plugins, if they need) would use it in `EventsToRegister` like this:
+
+```go
+// It means pods rejected by this plugin may become schedulable by the time flies.
+// isSchedulableAfterTimePasses is called periodically with rejected pods.
+{Event: fwk.ClusterEvent{Resource: fwk.Time}, QueueingHintFn: pl.isSchedulableAfterTimePasses}
+```
+
+At the scheduling queue, we'll have a new function `triggerTimeBasedQueueingHints`, which is triggered periodically, like `flushBackoffQCompleted`.
+In `triggerTimeBasedQueueingHints`, Queueing Hints with the `Resource: Type` event are triggered for pods rejected by those plugins,
+and the scheduling queue requeues/doesn't requeue pods based on QHints, as usual.
+
+`triggerTimeBasedQueueingHints` is triggered periodically, **but not very often**. Probably once 30 sec is enough.
+This is because:
+- Triggering `triggerTimeBasedQueueingHints` very often could impact the scheduling throughput because of the queue's lock.
+- Even if pods were requeued exactly after `ScaleupTimeout` passed, either way, those pods might have to wait for the backoff time to be completed, 
+and for other pods in activeQ to be handled. 
+
+For this reason, as you see in the above `ScaleupTimeout` comment, we would **not** guarantee that `ScaleupTimeout` means the pods are going to be retried exactly after the timeout period.
+
+As a summary, the `ScaleupTimeout` config will work like this:
+1. Pod with `ScaleUpFailed` in `FallbackCriteria` is rejected by the PodTopologySpread plugin.
+2. There's no cluster event that the PodTopologySpread plugin requeues the pod with.
+3. The cluster autoscaler somehow doesn't react to this pod. Maybe it's down.
+4. The scheduling queue triggers `triggerTimeBasedQueueingHints` periodically, and `triggerTimeBasedQueueingHints` invokes the PodTopologySpread plugin's QHint for `Resource: Type` event. 
+5. `ScaleupTimeout` is reached: the PodTopologySpread plugin's QHint for `Resource: Type` event returns `Queue` by comparing the pod's last scheduling time and `ScaleupTimeout`.
+6. The pod is retried, and the PodTopologySpread plugin regards TopologySpreadConstraint with `ScaleUpFailed` in `FallbackCriteria` as `ScheduleAnyway`. (fallback is triggered)
+
+
 #### How we implement `TriggeredScaleUp` in the cluster autoscaler
 
 Basically, we just put `TriggeredScaleUp: false` for Pods in [status.ScaleUpStatus.PodsRemainUnschedulable](https://github.com/kubernetes/autoscaler/blob/109998dbf30e6a6ef84fc37ebaccca23d7dee2f3/cluster-autoscaler/processors/status/scale_up_status_processor.go#L37) every [reconciliation (RunOnce)](https://github.com/kubernetes/autoscaler/blob/109998dbf30e6a6ef84fc37ebaccca23d7dee2f3/cluster-autoscaler/core/static_autoscaler.go#L296).