145 lines
4.8 KiB
Groff
145 lines
4.8 KiB
Groff
.\" $NetBSD: ctxsw.9,v 1.10 1999/03/16 00:40:47 garbled Exp $
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.\"
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.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Paul Kranenburg.
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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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd November 24, 1996
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.Dt CTXSW 9
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.Os
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.Sh NAME
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.Nm mi_switch ,
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.Nm cpu_switch
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.Nd switch to another process context
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.Sh SYNOPSIS
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.Ft void
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.Fn mi_switch "void"
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.Ft void
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.Fn cpu_switch "void"
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.Sh DESCRIPTION
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The
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.Fn mi_switch
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function implements the machine-independent prelude to a process context
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switch.
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It is called from only a few distinguished places in the kernel code as a
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result of the principle of non-preemptable kernel mode execution.
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The three major uses of
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.Fn mi_switch
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can be enumerated as follows:
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.Bl -enum -offset indent
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.It
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From within
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.Xr sleep 9
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and
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.Xr tsleep 9
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when the current process
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voluntarily relinquishes the CPU to wait for some resource to become
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available.
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.It
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After handling a trap
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.Pq e.g. a system call or device interrupt
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when the kernel prepares a return to user-mode execution.
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This case is typically handled by machine-dependent trap-handling code after
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detection of a change in the signal disposition of the current process, or
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when a higher priority process might be available to run.
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The latter event is communicated by the machine-independent scheduling
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routines by calling the machine-dependent
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.Fn need_resched "void" .
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.It
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In the signal handling code
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.Pq see Xr issignal 9
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if a signal is delivered that causes a process to stop.
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.El
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.Pp
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.Fn mi_switch
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records the amount of time the current process has been running in the
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process structure and checks this value against the CPU time limits
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allocated to the process
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.Pq see Xr getrlimit 2 .
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Exceeding the soft limit results in a
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.Dv SIGXCPU
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signal to be posted to the process, while exceeding the hard limit will
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cause a
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.Dv SIGKILL .
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After these administrative tasks are done,
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.Fn mi_switch
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hands over control to the machine dependent routine
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.Fn cpu_switch "void" ,
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which will perform the actual process context switch.
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.Pp
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.Fn cpu_switch
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will make a choice amongst the processes which are ready to run from a
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priority queue data-structure.
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The priority queue consists of an array
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.Va qs[NQS]
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of queue header structures each of which identifies a list of runnable
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processes of equal priority
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.Pq see Aq Pa sys/proc.h .
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A single word
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.Va whichqs
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containing a bit mask identifying non-empty queues assists in selecting
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a process quickly.
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.Fn cpu_switch
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must remove the first process from the list on the queue
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with the highest priority
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(lower indices in
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.Va qs
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indicate higher priority),
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and assign the address of its process structure to the global variable
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.Va curproc .
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If no processes are available on the run queues,
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.Fn cpu_switch
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shall go into an
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.Dq idle
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loop.
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The idle loop must allow interrupts to be taken that will eventually cause
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processes to appear again on the run queues.
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The variable
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.Va curproc
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should be
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.Dv NULL
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while
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.Fn cpu_switch
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waits for this to happen.
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.Pp
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Note that
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.Fn mi_switch
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and thus
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.Fn cpu_switch
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should be called at
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.Xr splhigh 9 .
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.Pp
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.Sh SEE ALSO
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.Xr tsleep 9 ,
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.Xr wakeup 9
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