.\"	$NetBSD: ctxsw.9,v 1.14 2002/09/27 08:38:58 wiz Exp $
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.\" by Paul Kranenburg.
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.Dd September 21, 2002
.Dt CTXSW 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switch
.Nd switch to another process context
.Sh SYNOPSIS
.Ft void
.Fn mi_switch "struct proc *curp" "struct proc *newp"
.Ft void
.Fn cpu_switch "struct proc *curp" "struct proc *newp"
.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
.Xr tsleep 9
when the current process voluntarily relinquishes the CPU to wait for
some resource to become available.
.It
From within
.Xr preempt 9
when the current process voluntarily relinquishes the CPU or when the
kernel prepares a return to user-mode execution.
.It
In the signal handling code
if a signal is delivered that causes a process to stop
.Pq see Xr issignal 9 .
.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 .
If
.Fa newp
is
.Dv NULL ,
.Fn mi_switch
will invoke
.Xr chooseproc 9
to select a new process from the system run queue.
If the new process is the same as the current process, then control is
returned immediately to the process, avoiding any unnecessary overhead
associated with switching in a new process.
Otherwise,
.Fn mi_switch
hands over control to the machine-dependent function
.Fn cpu_switch ,
which will perform the actual process context switch.
.Pp
The
.Fn cpu_switch
function switches the new process onto the CPU.
This procedure is performed by saving the process context of the current
process in its process control block (PCB) and restoring the process
context of the new process.
The address space of the new process is activated using
.Xr pmap_activate 9 .
Finally, the new process is checked to see if it was previously
preempted while inside a restartable atomic sequence
.Pq see Xr ras 9 .
.Pp
These functions must be called with the scheduler lock held
(see
.Xr SCHED_LOCK 9 )
and at the
.Xr splsched 9
interrupt protection level.
Both functions return with the scheduler lock released.
.Sh SEE ALSO
.Xr SCHED_LOCK 9 ,
.Xr chooseproc 9 ,
.Xr pmap_activate 9 ,
.Xr ras 9 ,
.Xr splsched 9 ,
.Xr tsleep 9 ,
.Xr wakeup 9