.\"     $NetBSD: cpu_need_resched.9,v 1.10 2019/11/23 19:46:38 ad Exp $
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.Dd November 17, 2019
.Dt CPU_NEED_RESCHED 9
.Os
.Sh NAME
.Nm cpu_need_resched
.Nd context switch notification
.Sh SYNOPSIS
.In sys/cpu.h
.Ft void
.Fn cpu_need_resched "struct cpu_info *ci" "struct lwp *l" "int flags"
.Sh DESCRIPTION
The
.Fn cpu_need_resched
function is the machine-independent interface for the scheduler to
notify machine-dependent code that a context switch from the current
LWP
.Fa l ,
on the cpu
.Fa ci ,
is required.
This event may occur if a higher priority LWP appears on the run
queue or if the current LWP has exceeded its time slice.
.Fa l
is the last LWP observed running on the CPU.
It may no longer be running, as
.Fn cpu_need_resched
can be called without holding scheduler locks.
.Pp
If the
.Dv RESCHED_KPREEMPT
flag is specified in
.Fa flags
and
.Dv __HAVE_PREEMPTION
C pre-processor macro is defined in
.In machine/intr.h ,
machine-dependent code should make a context switch happen as soon as possible
even if the CPU is running in kernel mode.
If the
.Dv RESCHED_KPREEMPT
flag is not specified, then
.Dv RESCHED_UPREEMPT
is specified instead.
.Pp
If the
.Dv RESCHED_IDLE
flag is specified in
.Fa flags ,
the last thread observed running on the CPU was the idle LWP.
.Pp
If
.Dv RESCHED_REMOTE
flag is specified in
.Fa flags ,
the request is not for the current CPU.
The opposite also holds true.
If
.Fa ci
is not the current processor,
.Fn cpu_need_resched
typically issues an inter processor call to the processor to make it
notice the need of a context switch as soon as possible.
.Pp
.Fn cpu_need_resched
is always called with kernel preemption disabled.
.Pp
Typically, the
.Fn cpu_need_resched
function will perform the following operations:
.Bl -bullet -offset indent
.It
Set a per-processor flag which is checked by
.Xr userret 9
when returning to user-mode execution.
.It
Post an asynchronous software trap
.Pq Tn AST .
.It
Send an inter processor interrupt to wake up
.Xr cpu_idle 9
and/or force an user process across the user/kernel boundary, thus making a
trip through
.Fn userret .
.El
.Sh SEE ALSO
.Xr sched_4bsd 9 ,
.Xr userret 9