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+.\"	$NetBSD: mi_switch.9,v 1.4 2017/07/03 21:28:48 wiz Exp $
+.\"
+.\" Copyright (c) 1996, 2002 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This code is derived from software contributed to The NetBSD Foundation
+.\" by Paul Kranenburg.
+.\"
+.\" 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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``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 FOUNDATION OR CONTRIBUTORS
+.\" 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.
+.\"
+.Dd July 21, 2007
+.Dt MI_SWITCH 9
+.Os
+.Sh NAME
+.Nm mi_switch
+.Nd machine independent context switch prelude
+.Sh SYNOPSIS
+.Ft int
+.Fn mi_switch "struct lwp *l"
+.Sh DESCRIPTION
+The
+.Fn mi_switch
+function implements the machine-independent prelude to an LWP context
+switch.
+It is called from only a few distinguished places in the kernel
+code as a result of the principle of non-preemptable kernel mode
+execution.
+The three major uses of
+.Fn mi_switch
+can be enumerated as follows:
+.Bl -enum -offset indent
+.It
+From within
+.Xr cv_wait 9
+and associated methods
+when the current LWP voluntarily relinquishes the CPU to wait for
+some resource to become available.
+.It
+From within
+.Xr preempt 9
+when the current LWP voluntarily relinquishes the CPU or when the
+kernel prepares a return to user-mode execution.
+.It
+In the signal handling code
+if a signal is delivered that causes an LWP to stop
+.Pq see Xr issignal 9 .
+.El
+.Pp
+.Fn mi_switch
+records the amount of time the current LWP has been running in the
+LWP structure and checks this value against the CPU time limits
+allocated to the LWP
+.Pq see Xr getrlimit 2 .
+Exceeding the soft limit results in a
+.Dv SIGXCPU
+signal to be posted to the LWP, while exceeding the hard limit will
+cause a
+.Dv SIGKILL .
+.Pp
+Unless
+.Fa l->l_switchto
+is not
+.Dv NULL ,
+.Fn mi_switch
+will call
+.Fn sched_nextlwp
+to select a new LWP from the scheduler's runqueue structures.
+If no runnable LWP is found, the idle LWP is used.
+If the new LWP is not equal to the current one,
+.Fn mi_switch
+will hand over control to the machine-dependent function
+.Xr cpu_switchto 9
+to switch to the new LWP.
+.Pp
+.Fn mi_switch
+has to be called with the LWP lock held
+(through calling
+.Fn lwp_lock
+first) and at the
+.Xr splsched 9
+interrupt protection level.
+It returns with the LWP lock released.
+.Sh RETURN VALUES
+.Fn mi_switch
+returns 1 if a context switch was performed to a
+different LWP, 0 otherwise.
+.Sh SEE ALSO
+.Xr condvar 9 ,
+.Xr cpu_switchto 9 ,
+.Xr csf 9 ,
+.Xr pmap 9 ,
+.Xr ras 9 ,
+.Xr sched_4bsd 9 ,
+.Xr splsched 9