NetBSD/share/man/man9/ctxsw.9

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.Dd July 8, 2006
.Dt CTXSW 9
.Os
.Sh NAME
.Nm mi_switch ,
.Nm cpu_switch ,
.Nm cpu_switchto
.Nd switch to another light weight process
.Sh SYNOPSIS
.Ft int
.Fn mi_switch "struct lwp *cur" "struct lwp *newl"
.Ft int
.Fn cpu_switch "struct lwp *cur" "struct lwp *newl"
.Ft int
.Fn cpu_switchto "struct lwp *cur" "struct lwp *newl"
.Sh DESCRIPTION
The
.Fn mi_switch
function implements the machine-independent prelude to an LWP
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 cv_wait 9
and associated methods
when the current LWP voluntarily relinquishes the CPU to wait for
some resource to become available.
.It
From within
.Xr preempt 9
when the current LWP 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 an LWP to stop
.Pq see Xr issignal 9 .
.El
.Pp
.Fn mi_switch
records the amount of time the current LWP has been running in the
LWP structure and checks this value against the CPU time limits
allocated to the LWP
.Pq see Xr getrlimit 2 .
Exceeding the soft limit results in a
.Dv SIGXCPU
signal to be posted to the LWP, while exceeding the hard limit will
cause a
.Dv SIGKILL .
If
.Fa newl
is
.Dv NULL ,
.Fn mi_switch
will hand over control to the machine-dependent function
.Fn cpu_switch
to select a new LWP from the system run queue.
If a new LWP is specified,
.Fn mi_switch
will hand over control to the machine-dependent function
.Fn cpu_switchto
to switch to the new LWP.
If the new LWP is the same as the current LWP, then control is
returned immediately to the LWP, avoiding any unnecessary overhead
associated with switching in a new LWP.
.Pp
The
.Fn cpu_switch
and
.Fn cpu_switchto
functions switch the new LWP onto the CPU.
This procedure is performed by saving the LWP context of the current
LWP in its process control block (PCB) and restoring the LWP
context of the new LWP.
The address space of the new LWP is activated using
.Fn pmap_activate
(see
.Xr pmap 9 ) .
Finally, the new LWP is checked to see if it was previously
preempted while inside a restartable atomic sequence
.Pq see Xr ras 9 .
.Pp
.Fn mi_switch
has to be called with the LWP lock held
(through calling 
.Fn lwp_lock
first) and at the
.Xr splsched 9
interrupt protection level. It returns with the LWP lock released.
.Sh RETURN VALUES
These functions return 1 if a context switch was performed to a
different LWP, 0 otherwise.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr pmap 9 ,
.Xr ras 9 ,
.Xr splsched 9