111 lines
3.5 KiB
Groff
111 lines
3.5 KiB
Groff
.\" $NetBSD: mi_switch.9,v 1.2 2008/04/30 13:10:58 martin 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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.\"
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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 July 21, 2007
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.Dt MI_SWITCH 9
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.Os
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.Sh NAME
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.Nm mi_switch
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.Nd machine independent context switch prelude
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.Sh SYNOPSIS
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.Ft int
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.Fn mi_switch "struct lwp *l"
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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 context
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switch. It is called from only a few distinguished places in the
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kernel code as a result of the principle of non-preemptable kernel
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mode execution. 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 cv_wait 9
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and associated methods
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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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.Pp
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Unless
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.Fa l->l_switchto
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is not
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.Dv NULL ,
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.Fn mi_switch
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will call
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.Fn sched_nextlwp
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to select a new LWP from the scheduler's runqueue structures. If no
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runnable LWP is found, the idle LWP is used.
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If the new LWP is not equal to the current one,
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.Fn mi_switch
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will hand over control to the machine-dependent function
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.Xr cpu_switchto 9
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to switch to the new LWP.
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.Pp
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.Fn mi_switch
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has to be called with the LWP lock held
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(through calling
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.Fn lwp_lock
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first) and at the
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.Xr splsched 9
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interrupt protection level. It returns with the LWP lock released.
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.Sh RETURN VALUES
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.Fn mi_switch
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returns 1 if a context switch was performed to a
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different LWP, 0 otherwise.
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.Sh SEE ALSO
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.Xr condvar 9 ,
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.Xr cpu_switchto 9 ,
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.Xr csf 9 ,
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.Xr pmap 9 ,
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.Xr ras 9 ,
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.Xr sched_4bsd 9 ,
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.Xr splsched 9
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