freebsd-dev/share/man/man9/runqueue.9

.\" Copyright (c) 2000-2001 John H. Baldwin <jhb@FreeBSD.org>
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd November 3, 2000
.Dt RUNQUEUE 9
.Os
.Sh NAME
.Nm chooseproc ,
.Nm procrunnable ,
.Nm remrunqueue ,
.Nm setrunqueue
.Nd manage the queue of runnable processes
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
.Vt "extern	struct rq itqueues[]" ;
.Vt "extern	struct rq rtqueues[]" ;
.Vt "extern	struct rq queues[]" ;
.Vt "extern	struct rq idqueues[]" ;
.Ft struct thread *
.Fn choosethread "void"
.Ft int
.Fn procrunnable "void"
.Ft void
.Fn remrunqueue "struct thread *td"
.Ft void
.Fn setrunqueue "struct thread *td"
.Sh DESCRIPTION
The run queue consists of four priority queues:
.Va itqueues
for interrupt threads,
.Va rtqueues
for realtime priority processes,
.Va queues
for time sharing processes, and
.Va idqueues
for idle priority processes.
Each priority queue consists of an array of
.Dv 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
.Va itqueuebits ,
.Va rtqueuebits ,
.Va queuebits ,
and
.Va idqueuebits .
The run queues are protected by the
.Va sched_lock
mutex.
.Pp
.Fn 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
.Va sched_lock
mutex does
.Em 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
.Fn procrunnable
may have to require that the
.Va sched_lock
mutex be acquired.
.Pp
.Fn choosethread
returns the highest priority runnable thread.
If there are no runnable threads, then the idle thread is returned.
This function is called by
.Fn cpu_switch
and
.Fn cpu_throw
to determine which thread to switch to.
.Fn choosethread
must be called with the
.Va sched_lock
mutex held.
.Pp
.Fn setrunqueue
adds the thread
.Fa td
to the tail of the appropriate queue in the proper priority queue.
The thread must be runnable, i.e.\&
.Va p_stat
must be set to
.Dv SRUN .
This function must be called with the
.Va sched_lock
mutex held.
.Pp
.Fn remrunqueue
removes thread
.Fa td
from its run queue.
If
.Fa td
is not on a run queue, then the kernel will
.Xr panic 9 .
This function must be called with the
.Va sched_lock
mutex held.
.Sh SEE ALSO
.Xr cpu_switch 9 ,
.Xr scheduler 9 ,
.Xr sleepqueue 9