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NAME

     chooseproc, 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)