Copyright (c) 1996 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.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the NetBSD
Foundation, Inc. and its contributors.
4. Neither the name of The NetBSD Foundation nor the names of its
contributors may be used to endorse or promote products derived
from this software without specific prior written permission.
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 REGENTS 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 Nov 24, 1996 .Dt mi_switch 9 .Os FreeBSD .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-preemtable kernel mode execution. The three major uses of .Nm can be enumerated as follows: l -enum -offset indent t 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. t after handling a trap
q e.g. a system call, 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 defined .Fn need_resched "void" . t in the signal handling code
q see Xr issignal 9 if a signal is delivered that causes a process to stop. .El
p .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
q 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 . 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.
p .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
q see Fa <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
o lower indices in Va sq indicate higher priority
c , and assign the address of its process structure to the global variable .Dv 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.
p Note that .Fn mi_switch and thus .Fn cpu_switch should be called at splhigh().
p .Sh SEE ALSO .Xr issignal 9 , .Xr spl 9 , .Xr tsleep 9 , .Xr wakeup 9
p