wake_up_all() is only needed when there are threads doing an exclusive wait.

wake_up() will wake up all non-exclusive threads, and at most one exclusive thread.  When non-exclusive waits are used, it is sufficient and preferred to use wake_up() to  wake them up.

wake_up_all() should be reserved for those cases where threads are waiting exclusively, and they all want to be woken up.  Keeping it for the purpose only improve code readability.