to awaken all waiters when a contested mutex is released instead of just
the highest priority waiter. If the various threads are awakened in
sequence then each thread may acquire and release the lock in question
without contention resulting in fewer expensive unlock and lock
operations. This old behavior of waking just the highest priority is
still used if this option is specified. Making the algorithm conditional
on a kernel option will allows us to benchmark both cases later and
determine which one should be used by default.
Requested by: tanimura-san
535eb30962