.\"     $NetBSD: cpu_idle.9,v 1.3 2003/04/16 13:35:26 wiz Exp $
.\"
.\" Copyright (c) 2002 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Gregory McGarry.
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.Dd September 21, 2002
.Dt CPU_IDLE 9
.Os
.Sh NAME
.Nm cpu_idle
.Nd idle CPU while waiting for work
.Sh SYNOPSIS
.In sys/proc.h
.Ft void
.Fn cpu_idle "void"
.Sh DESCRIPTION
The
.Fn cpu_idle
function is invoked to idle the current CPU while waiting for work to be done.
It is primarily called by the scheduler if no processes are
available on the system run queue.
The
.Fn cpu_idle
function is a machine-dependent implementations in order to permit
low-level CPU control.
.Pp
The
.Fn cpu_idle
function must be called with the scheduler lock held
.P see Xr SCHED_LOCK 9
and at the
.Xr splsched 9
interrupt protection level.
It performs the following operations:
.Bl -bullet
.It
Release the scheduler lock.
.It
Lower the current interrupt priority level to permit interrupts to be
taken that will eventually cause processes to appear again on the
system run queue.
.It
Try to zero some pages for the virtual memory system.
.It
Perform any CPU or system power control.
.It
Wait until a process appears on the system run queue.
.It
Acquire the scheduler lock.
.El
.Pp
For CPUs which don't support an idle operation, the CPU must spin
until a process appears on the system run queue.
Generally, this is done by checking if the
.Va sched_whichqs
variable in the scheduler is not zero.
See
.Xr setrunqueue 9
for further details.
.Sh SEE ALSO
.Xr mi_switch 9 ,
.Xr scheduler 9 ,
.Xr setrunqueue 9 ,
.Xr spl 9 ,
.Xr uvm 9