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159 lines
4.9 KiB
Groff
159 lines
4.9 KiB
Groff
.\" $NetBSD: ctxsw.9,v 1.2 1996/12/02 00:11:31 tls Exp $
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.\"
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.\" Copyright (c) 1996 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 REGENTS OR CONTRIBUTORS BE
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.\" 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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.\" $FreeBSD$
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.\"
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.Dd November 24, 1996
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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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.Nm cpu_switch ,
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.Nm cpu_throw
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.Nd switch to another thread context
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/proc.h
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.Ft void
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.Fn mi_switch "void"
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.Ft void
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.Fn cpu_switch "void"
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.Ft void
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.Fn cpu_throw "void"
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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 a thread context
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switch.
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It is called from only a few distinguished places in the kernel
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code as a result of the principle of non-preemptable kernel mode execution.
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The various major uses of
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.Nm
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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 a function such as
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.Xr cv_wait 9 ,
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.Xr mtx_lock ,
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or
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.Xr tsleep 9
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when the current thread
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voluntarily relinquishes the CPU to wait for some resource or lock to become
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available.
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.It
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After handling a trap
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(e.g.\& a system call, device interrupt)
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when the kernel prepares a return to user-mode execution.
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This case is
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typically handled by machine dependent trap-handling code after detection
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of a change in the signal disposition of the current process, or when a
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higher priority thread might be available to run.
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The latter event is
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communicated by the machine independent scheduling routines by calling
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the machine defined
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.Fn need_resched .
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.It
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In the signal handling code
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(see
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.Xr issignal 9 )
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if a signal is delivered that causes a process to stop.
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.It
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When a thread dies in
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.Xr thread_exit 9
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and control of the processor can be passed to the next runnable thread.
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.It
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In
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.Xr thread_suspend_check 9
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where a thread needs to stop execution due to the suspension state of
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the process as a whole.
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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 thread has been running in the
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process structures and checks this value against the CPU time limits
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allocated to the process
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(see
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.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 process, 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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If the thread is still in the
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.Dv TDS_RUNNING
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state,
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.Fn mi_switch
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will put it back onto the run queue, assuming that
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it will want to run again soon.
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If it is in one of the other
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states and KSE threading is enabled, the associated
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.Em KSE
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will be made available to any higher priority threads from the same
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group, to allow them to be scheduled next.
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.Pp
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After these administrative tasks are done,
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.Fn mi_switch
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hands over control to the machine dependent routine
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.Fn cpu_switch ,
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which will perform the actual thread context switch.
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.Pp
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.Fn cpu_switch
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first saves the context of the current thread.
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Next, it calls
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.Fn choosethread
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to determine which thread to run next.
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Finally, it reads in the saved context of the new thread and starts to
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execute the new thread.
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.Pp
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.Fn cpu_throw
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is similar to
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.Fn cpu_switch
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except that it does not save the context of the old thread.
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This function is useful when the kernel does not have an old thread
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context to save, such as when CPUs other than the boot CPU perform their
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first task switch, or when the kernel does not care about the state of the
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old thread, such as in
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.Fn thread_exit
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when the kernel terminates the current thread and switches into a new
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thread.
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.Pp
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To protect the
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.Xr runqueue 9 ,
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all of these functions must be called with the
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.Va sched_lock
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mutex held.
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.Sh SEE ALSO
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.Xr cv_wait 9 ,
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.Xr issignal 9 ,
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.Xr mutex 9 ,
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.Xr runqueue 9 ,
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.Xr tsleep 9 ,
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.Xr wakeup 9
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