.\"	$OpenBSD: ctxsw.9,v 1.7 2000/12/21 18:15:14 avsm Exp $
.\"	$NetBSD: ctxsw.9,v 1.9 1999/03/06 22:09:29 mycroft Exp $
.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
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.\"        This product includes software developed by the NetBSD
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.\"    contributors may be used to endorse or promote products derived
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.Dd November 24, 1996
.Dt CTXSW 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switch
.Nd switch to another process context
.Sh SYNOPSIS
.Ft void
.Fn mi_switch "void"
.Ft void
.Fn cpu_switch "void"
.Sh DESCRIPTION
The
.Fn mi_switch
function implements the machine-independent prelude to a process 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 sleep 9
and
.Xr tsleep 9
when the current process
voluntarily relinquishes the CPU to wait for some resource to become
available.
.It
After handling a trap
.Pq e.g., a system call or device interrupt
when the kernel prepares a return to user-mode execution.
This case is typically handled by machine-dependent trap-handling code after
detection of a change in the signal disposition of the current process, or
when a higher priority process might be available to run.
The latter event is communicated by the machine-independent scheduling
routines by calling the machine-dependent
.Fn need_resched "void" .
.It
In the signal handling code
.Pq see Xr issignal 9
if a signal is delivered that causes a process to stop.
.El
.Pp
.Fn mi_switch
records the amount of time the current process has been running in the
process structure and checks this value against the CPU time limits
allocated to the process
.Pq see Xr getrlimit 2 .
Exceeding the soft limit results in a
.Dv SIGXCPU
signal to be posted to the process, while exceeding the hard limit will
cause a
.Dv SIGKILL .
For a process which accumulated longer than 10 minutes of
cpu time, its nice level is raised to 4.
After these administrative tasks are done,
.Fn mi_switch
hands over control to the machine dependent routine
.Fn cpu_switch "void" ,
which will perform the actual process context switch.
.Pp
.Fn cpu_switch
will make a choice amongst the processes which are ready to run from a
priority queue data-structure.
The priority queue consists of an array
.Va qs[NQS]
of queue header structures each of which identifies a list of runnable
processes of equal priority
.Pq see Aq Pa sys/proc.h .
A single word
.Va whichqs
containing a bit mask identifying non-empty queues assists in selecting
a process quickly.
.Fn cpu_switch
must remove the first process from the list on the queue
with the highest priority
(lower indices in
.Va qs
indicate higher priority),
and assign the address of its process structure to the global variable
.Va curproc .
If no processes are available on the run queues,
.Fn cpu_switch
shall go into an
.Dq idle
loop.
The idle loop must allow interrupts to be taken that will eventually cause
processes to appear again on the run queues.
The variable
.Va curproc
should be
.Dv NULL
while
.Fn cpu_switch
waits for this to happen.
.Pp
Note that
.Fn mi_switch
and thus
.Fn cpu_switch
should be called at
.Xr splhigh 9 .
.Pp
.Sh SEE ALSO
.Xr spl 9 ,
.Xr tsleep 9 ,
.Xr wakeup 9