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 ofNQS 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.