NAME

     choosethread, procrunnable, remrunqueue, setrunqueue — manage the queue of runnable
     processes

SYNOPSIS

     #include <sys/param.h>
     #include <sys/proc.h>

     extern struct rq itqueues[];
     extern struct rq rtqueues[];
     extern struct rq queues[];
     extern struct rq idqueues[];

     struct thread *
     choosethread(void);

     int
     procrunnable(void);

     void
     remrunqueue(struct thread *td);

     void
     setrunqueue(struct thread *td);

DESCRIPTION

     The run queue consists of four priority queues: itqueues for interrupt threads, rtqueues for
     realtime priority processes, queues for time sharing processes, and idqueues for idle
     priority processes.  Each priority queue consists of an array of NQS queue header
     structures.  Each queue header identifies a list of runnable processes of equal priority.
     Each queue also has a single word that contains a bit mask identifying non-empty queues to
     assist in selecting a process quickly.  These are named itqueuebits, rtqueuebits, queuebits,
     and idqueuebits.  The run queues are protected by the sched_lock mutex.

     procrunnable() returns zero if there are no runnable processes other than the idle process.
     If there is at least one runnable process other than the idle process, it will return a non-
     zero value.  Note that the sched_lock mutex does not need to be held when this function is
     called.  There is a small race window where one CPU may place a process on the run queue
     when there are currently no other runnable processes while another CPU is calling this
     function.  In that case the second CPU will simply travel through the idle loop one
     additional time before noticing that there is a runnable process.  This works because idle
     CPUs are not halted in SMP systems.  If idle CPUs are halted in SMP systems, then this race
     condition might have more serious repercussions in the losing case, and procrunnable() may
     have to require that the sched_lock mutex be acquired.

     choosethread() returns the highest priority runnable thread.  If there are no runnable
     threads, then the idle thread is returned.  This function is called by cpu_switch() and
     cpu_throw() to determine which thread to switch to.  choosethread() must be called with the
     sched_lock mutex held.

     setrunqueue() adds the thread td to the tail of the appropriate queue in the proper priority
     queue.  The thread must be runnable, i.e. p_stat must be set to SRUN.  This function must be
     called with the sched_lock mutex held.

     remrunqueue() removes thread td from its run queue.  If td is not on a run queue, then the
     kernel will panic(9).  This function must be called with the sched_lock mutex held.

SEE ALSO

     cpu_switch(9), scheduler(9), sleepqueue(9)