the current implementation does the following to 'give up' the CPU:

   - it decreases its priority by 1 until it reaches the lowest level
   - it queues the task to the end of the priority queue

  this scheme works fine in most cases, but if sched_yield()-active tasks
  are mixed with CPU-using processes then it's quite likely that the
  CPU-using process is in the expired array. In that case the yield()-ing
  process only requeues itself in the active array - a true context-switch
  to the expired process will only occur once the timeslice of the
  yield()-ing process has expired: in ~150 msecs. This leads to the
  yield()-ing and CPU-using process to use up rougly the same amount of
  CPU-time, which is arguably deficient.

  i've fixed this problem by extending sched_yield() the following way:

  +        * There are three levels of how a yielding task will give up
  +        * the current CPU:
  +        *
  +        *  #1 - it decreases its priority by one. This priority loss is
  +        *       temporary, it's recovered once the current timeslice
  +        *       expires.
  +        *
  +        *  #2 - once it has reached the lowest priority level,
  +        *       it will give up timeslices one by one. (We do not
  +        *       want to give them up all at once, it's gradual,
  +        *       to protect the casual yield()er.)
  +        *
  +        *  #3 - once all timeslices are gone we put the process into
  +        *       the expired array.
  +        *
  +        *  (special rule: RT tasks do not lose any priority, they just
  +        *  roundrobin on their current priority level.)
  +        */