.\"	$NetBSD: ctxsw.9,v 1.2 1996/12/02 00:11:31 tls Exp $
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.\" by Paul Kranenburg.
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.Dd November 24, 1996
.Dt MI_SWITCH 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switch ,
.Nm cpu_throw
.Nd switch to another process context
.Sh SYNOPSIS
.Fd #include <sys/param.h>
.Fd #include <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 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:
.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
(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 .
.It
in the signal handling code
(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
(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 ,
which will perform the actual process context switch.
.Pp
.Fn cpu_switch
first saves the context of the current process.
Next, it calls
.Fn chooseproc
to determine which process to run next.
Finally, it reads in the saved context of the new process and starts to
execute the new process.
.Pp
.Fn cpu_throw
is similar to
.Fn cpu_switch
except that it does not save the context of the old process.
This function is useful when the kernel does not have an old process
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 process, such as in
.Fn cpu_exit
when the kernel terminates the current process and switches into a new
process.
.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 issignal 9 ,
.Xr mutex 9 ,
.Xr runqueue 9 ,
.Xr tsleep 9 ,
.Xr wakeup 9