.\"	$OpenBSD: mi_switch.9,v 1.7 2017/09/05 03:49:56 jmatthew Exp $
.\"	$NetBSD: ctxsw.9,v 1.9 1999/03/06 22:09:29 mycroft Exp $
.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
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.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Paul Kranenburg.
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.Dd $Mdocdate: September 5 2017 $
.Dt MI_SWITCH 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switchto
.Nd switch to another process context
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
.Ft void
.Fn mi_switch "void"
.Ft void
.Fn cpu_switchto "struct proc *old" "struct proc *new"
.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 functions like
.Xr tsleep 9 ,
when the current process sleeps to wait for some resource to become
available, or from within functions like
.Fn yield ,
when it relinquishes the CPU to allow other processes to make progress.
.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 Pa sys/kern/kern_sig.c
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 .
After these administrative tasks are done,
.Fn mi_switch
chooses the next process to run and hands over control to the machine
dependent routine
.Fn cpu_switchto ,
which will perform the actual process context switch.
.Pp
.Fn cpu_switchto
will save the context of the old process and switch to the new one.
A special case is when the old process is
.Dv NULL
which means that the old process has exited and doesn't need to be
saved.
.Pp
Note that
.Fn mi_switch
and thus
.Fn cpu_switchto
must be called while holding the scheduler lock.
.Sh SEE ALSO
.Xr spl 9 ,
.Xr tsleep 9 ,
.Xr wakeup 9