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.Dd July 21, 2007
.Dt MI_SWITCH 9
.Os
.Sh NAME
.Nm mi_switch
.Nd machine independent context switch prelude
.Sh SYNOPSIS
.Ft int
.Fn mi_switch "struct lwp *l"
.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 .
.Pp
Unless
.Fa l->l_switchto
is not
.Dv NULL ,
.Fn mi_switch
will call
.Fn sched_nextlwp
to select a new LWP from the scheduler's runqueue structures.
If no runnable LWP is found, the idle LWP is used.
If the new LWP is not equal to the current one,
.Fn mi_switch
will hand over control to the machine-dependent function
.Xr cpu_switchto 9
to switch to the new LWP.
.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
.Fn mi_switch
returns 1 if a context switch was performed to a
different LWP, 0 otherwise.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr cpu_switchto 9 ,
.Xr csf 9 ,
.Xr pmap 9 ,
.Xr ras 9 ,
.Xr sched_4bsd 9 ,
.Xr splsched 9