135 lines
4.5 KiB
Groff
135 lines
4.5 KiB
Groff
.\" $NetBSD: ctxsw.9,v 1.17 2003/05/13 09:59:27 wiz Exp $
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.\"
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.\" Copyright (c) 1996, 2002 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 May 13, 2003
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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 light weight process
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.Sh SYNOPSIS
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.Ft int
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.Fn mi_switch "struct lwp *cur" "struct lwp *new"
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.Ft int
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.Fn cpu_switch "struct lwp *cur" "struct lwp *new"
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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 an LWP
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context switch.
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It is called from only a few distinguished places in the kernel code
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as a 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 tsleep 9
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when the current LWP voluntarily relinquishes the CPU to wait for
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some resource to become available.
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.It
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From within
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.Xr preempt 9
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when the current LWP voluntarily relinquishes the CPU or when the
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kernel prepares a return to user-mode execution.
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.It
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In the signal handling code
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if a signal is delivered that causes an LWP to stop
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.Pq see Xr issignal 9 .
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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 LWP has been running in the
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LWP structure and checks this value against the CPU time limits
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allocated to the LWP
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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 LWP, while exceeding the hard limit will
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cause a
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.Dv SIGKILL .
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If
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.Fa newp
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is
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.Dv NULL ,
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.Fn mi_switch
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will invoke
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.Xr chooseproc 9
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to select a new LWP from the system run queue.
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If the new LWP is the same as the current LWP, then control is
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returned immediately to the LWP, avoiding any unnecessary overhead
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associated with switching in a new LWP.
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Otherwise,
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.Fn mi_switch
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hands over control to the machine-dependent function
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.Fn cpu_switch ,
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which will perform the actual LWP context switch.
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.Pp
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The
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.Fn cpu_switch
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function switches the new LWP onto the CPU.
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This procedure is performed by saving the LWP context of the current
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LWP in its process control block (PCB) and restoring the LWP
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context of the new LWP.
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The address space of the new LWP is activated using
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.Fn pmap_activate
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(see
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.Xr pmap 9 ) .
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Finally, the new LWP is checked to see if it was previously
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preempted while inside a restartable atomic sequence
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.Pq see Xr ras 9 .
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.Pp
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These functions must be called with the scheduler lock held
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(see
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.Xr SCHED_LOCK 9 )
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and at the
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.Xr splsched 9
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interrupt protection level.
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Both functions return with the scheduler lock released.
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.Sh RETURN VALUES
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These functions return 1 if a context switch was performed to a different
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LWP, 0 otherwise.
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.Sh SEE ALSO
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.Xr SCHED_LOCK 9 ,
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.Xr chooseproc 9 ,
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.Xr pmap 9 ,
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.Xr ras 9 ,
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.Xr splsched 9 ,
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.Xr tsleep 9 ,
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.Xr wakeup 9
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