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RUNQUEUE(9)		 BSD Kernel Developer's	Manual		   RUNQUEUE(9)

     chooseproc, procrunnable, remrunqueue, setrunqueue	-- manage the queue of
     runnable processes

     #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 *


     remrunqueue(struct	thread *td);

     setrunqueue(struct	thread *td);

     The run queue consists of four priority queues: itqueues for interrupt
     threads, rtqueues for realtime priority processes,	queues for time	shar-
     ing 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

     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.

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

BSD			       November	3, 2000				   BSD


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