tokio::task::yield_now

yield_now を呼んだからといって必ず他のタスクに切り替わるという保証はない。

ランタイム次第では、同じタスクにすぐ戻ってきてしまうこともある

the runtime may choose to poll the task that just ran yield_now() again immediately without polling any other tasks first.

For example, the runtime will not drive the IO driver between every poll of a task, and this could result in the runtime polling the current task again immediately even if there is another task that could make progress if that other task is waiting for a notification from the IO driver.