Add feature flag treat-pod-as-always-schedulable

[SaschaSchwarze0]

The feature flag allows to declare that Pods in the system will eventually all get scheduled and Revisions should therefore not be marked unschedulable

Fixes #14862

Proposed Changes

Unfortunately, I never got feedback in above issue, so trying with a PR now:

• New feature treat-pod-as-always-schedulable is added with default Disabled.
• If enabled, the revision reconciler will skip the check whether a pod is not scheduled and therefore never mark a revision as unschedulable.

Release Note

You can now set the new feature `treat-pod-as-always-schedulable` to enabled in the config-defaults ConfigMap. As a result, Knative will not mark revisions as Unschedulable when a Pod is not scheduled. This makes sense if you want to omit this transient state in clusters that have cluster-autoscaling set up, and you can guarantee that all Pods will be eventually scheduled.

[knative-prow]

Hi @SaschaSchwarze0. Thanks for your PR.

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[SaschaSchwarze0]

Hm, I debugged the failing unit test and can see that when the code checks the config, that there is some config in the context with the right value but somehow what it gets has it empty. Need to check ...

[skonto]

@SaschaSchwarze0 gentle ping.

[skonto]

/ok-to-test

[knative-prow]

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[SaschaSchwarze0]

@SaschaSchwarze0 gentle ping.

Rebased, but I still do not understand the unit test case failure in pkg/reconciler/revision/table_test.go. In particular, the custom context with the custom feature flag does not work = the code does not get the expected feature flag status and I do not understand why.

EDIT1: I think I now understand it. table_test.go does the following which overwrites all config customizations done on the Ctx of the row.

		return revisionreconciler.NewReconciler(ctx, logging.FromContext(ctx), servingclient.Get(ctx),
			listers.GetRevisionLister(), controller.GetEventRecorder(ctx), r,
			controller.Options{
				ConfigStore: &testConfigStore{
					config: reconcilerTestConfig(),
				},
			})

EDIT2: Though, I do not understand why it works for other unit tests then (like the one that sets PodSpecInitContainers to Enabled).

EDIT3: Maybe because only the controller uses the configstore from its options.

EDIT4: Fixed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.84%. Comparing base (19b9a09) to head (5303dc0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15397      +/-   ##
==========================================
+ Coverage   80.78%   80.84%   +0.06%     
==========================================
  Files         222      222              
  Lines       18025    18029       +4     
==========================================
+ Hits        14561    14576      +15     
+ Misses       3092     3084       -8     
+ Partials      372      369       -3     

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