addUnschedulablePodBackToBackoffQ happened to put the pod into the backoff
queue because
- the pod was not popped earlier and thus not in flight
- the PodInfo had UnschedulablePlugins set
- determineSchedulingHintForInFlightPod has code for "if UnschedulablePlugins
is set and pod not in flight -> internal error, use backoff"
Relying on such special code is not good. A better way to force backoff is by
recording some concurrent event. isPodWorthRequeuing then calls the
queueHintReturnQueueAfterBackoff function and the pod goes to the backoff
queue.
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