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 $
|
|
.\"
|
|
.\" 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.
|
|
.\"
|
|
.\" 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.
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd November 24, 1996
|
|
.Dt MI_SWITCH 9
|
|
.Os
|
|
.Sh NAME
|
|
.Nm mi_switch ,
|
|
.Nm cpu_switch ,
|
|
.Nm cpu_throw
|
|
.Nd switch to another thread context
|
|
.Sh SYNOPSIS
|
|
.In sys/param.h
|
|
.In sys/proc.h
|
|
.Ft void
|
|
.Fn mi_switch "void"
|
|
.Ft void
|
|
.Fn cpu_switch "void"
|
|
.Ft void
|
|
.Fn cpu_throw "void"
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Fn mi_switch
|
|
function implements the machine independent prelude to a thread 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 various major uses of
|
|
.Nm
|
|
can be enumerated as follows:
|
|
.Bl -enum -offset indent
|
|
.It
|
|
From within a function such as
|
|
.Xr cv_wait 9 ,
|
|
.Xr mtx_lock 9 ,
|
|
or
|
|
.Xr tsleep 9
|
|
when the current thread
|
|
voluntarily relinquishes the CPU to wait for some resource or lock to become
|
|
available.
|
|
.It
|
|
After handling a trap
|
|
(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 thread might be available to run.
|
|
The latter event is
|
|
communicated by the machine independent scheduling routines by calling
|
|
the machine defined
|
|
.Fn need_resched .
|
|
.It
|
|
In the signal handling code
|
|
(see
|
|
.Xr issignal 9 )
|
|
if a signal is delivered that causes a process to stop.
|
|
.It
|
|
When a thread dies in
|
|
.Xr thread_exit 9
|
|
and control of the processor can be passed to the next runnable thread.
|
|
.It
|
|
In
|
|
.Xr thread_suspend_check 9
|
|
where a thread needs to stop execution due to the suspension state of
|
|
the process as a whole.
|
|
.El
|
|
.Pp
|
|
.Fn mi_switch
|
|
records the amount of time the current thread has been running in the
|
|
process structures and checks this value against the CPU time limits
|
|
allocated to the process
|
|
(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 .
|
|
.Pp
|
|
If the thread is still in the
|
|
.Dv TDS_RUNNING
|
|
state,
|
|
.Fn mi_switch
|
|
will put it back onto the run queue, assuming that
|
|
it will want to run again soon.
|
|
If it is in one of the other
|
|
states and KSE threading is enabled, the associated
|
|
.Em KSE
|
|
will be made available to any higher priority threads from the same
|
|
group, to allow them to be scheduled next.
|
|
.Pp
|
|
After these administrative tasks are done,
|
|
.Fn mi_switch
|
|
hands over control to the machine dependent routine
|
|
.Fn cpu_switch ,
|
|
which will perform the actual thread context switch.
|
|
.Pp
|
|
.Fn cpu_switch
|
|
first saves the context of the current thread.
|
|
Next, it calls
|
|
.Fn choosethread
|
|
to determine which thread to run next.
|
|
Finally, it reads in the saved context of the new thread and starts to
|
|
execute the new thread.
|
|
.Pp
|
|
.Fn cpu_throw
|
|
is similar to
|
|
.Fn cpu_switch
|
|
except that it does not save the context of the old thread.
|
|
This function is useful when the kernel does not have an old thread
|
|
context to save, such as when CPUs other than the boot CPU perform their
|
|
first task switch, or when the kernel does not care about the state of the
|
|
old thread, such as in
|
|
.Fn thread_exit
|
|
when the kernel terminates the current thread and switches into a new
|
|
thread.
|
|
.Pp
|
|
To protect the
|
|
.Xr runqueue 9 ,
|
|
all of these functions must be called with the
|
|
.Va sched_lock
|
|
mutex held.
|
|
.Sh SEE ALSO
|
|
.Xr cv_wait 9 ,
|
|
.Xr issignal 9 ,
|
|
.Xr mutex 9 ,
|
|
.Xr runqueue 9 ,
|
|
.Xr tsleep 9 ,
|
|
.Xr wakeup 9
|