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Document the current organization of the run queues and the various
functions used to manage them. Reviewed by: sheldonh
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John Baldwin
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share/man/man9/runqueue.9
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share/man/man9/runqueue.9
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.\" Copyright (c) 2000 John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd Nov 3, 2000
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.Dt RUNQUEUE 9
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.Os
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.Sh NAME
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.Nm chooseproc ,
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.Nm procrunnable ,
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.Nm remrunqueue ,
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.Nm setrunqueue
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.Nd manage the queue of runnable processes
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.Sh SYNOPSIS
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.Fd #include <sys/param.h>
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.Fd #include <sys/proc.h>
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.Vt extern struct rq itqueues[];
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.Vt extern struct rq rtqueues[];
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.Vt extern struct rq queues[];
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.Vt extern struct rq idqueues[];
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.Ft struct proc *
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.Fn chooseproc "void"
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.Ft u_int32_t
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.Fn procrunnable "void"
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.Ft void
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.Fn remrunqueue "struct proc *p"
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.Ft void
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.Fn setrunqueue "struct proc *p"
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.Sh DESCRIPTION
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The run queue consists of four priority queues:
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.Va itqueues
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for interrupt threads,
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.Va rtqueues
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for realtime priority processes,
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.Va queues
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for time sharing processes, and
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.Va idqueues
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for idle priority processes.
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Each priority queue consists of an array of
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.Dv NQS
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queue header structures.
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Each queue header identifies a list of runnable processes of equal priority.
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Each queue also has a single word that contains a bit mask identifying
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non-empty queues to assist in selecting a process quickly.
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These are named
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.Va itqueuebits ,
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.Va rtqueuebits ,
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.Va queuebits ,
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and
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.Va idqueuebits .
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The run queues are protected by the
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.Va sched_lock
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mutex.
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.Pp
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.Fn procrunnable
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returns zero if there are no runnable processes other than the idle process.
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If there is at least one runnable process other than the idle process, it
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will return a non-zero value.
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Note that the
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.Va sched_lock
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mutex does
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.Em not
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need to be held when this function is called.
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There is a small race window where one CPU may place a process on the run queue
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when there are currently no other runnable processes while another CPU is
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calling this function.
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In that case the second CPU will simply travel through the idle loop one
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additional time before noticing that there is a runnable process.
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This works because idle CPUs are not halted in SMP systems.
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If idle CPUs are halted in SMP systems, then this race condition might have
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more serious repurcussions in the losing case, and
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.Fn procrunnable
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may have to require that the
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.Va sched_lock
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mutex be acquired.
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.Pp
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.Fn chooseproc
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returns the highest priority runnable process.
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If there are no runnable processes, then the idle process is returned.
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This function is called by
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.Fn cpu_switch
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and
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.Fn cpu_throw
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to determine which process to switch to.
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.Fn chooseproc
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must be called with the
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.Va sched_lock
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mutex held.
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.Pp
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.Fn setrunqueue
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adds the process
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.Fa p
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to the tail of the appropriate queue in the proper priority queue.
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The process must be runnable, i.e.
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.Va p_stat
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must be set to
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.Dv SRUN .
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This function must be called with the
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.Va sched_lock
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mutex held.
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.Pp
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.Fn remrunqueue
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removes process
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.Fa p
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from its run queue.
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If
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.Fa p
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is not on a run queue, then the kernel will
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.Xr panic 9 .
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This function must be called with the
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.Va sched_lock
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mutex held.
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.Sh SEE ALSO
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.Xr cpu_switch 9 ,
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.Xr scheduler 9 ,
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.Xr sleepqueue 9
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