NAME
mi_switch —
machine independent context
switch prelude
SYNOPSIS
int
mi_switch(
struct
lwp *l);
DESCRIPTION
The
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
mi_switch()
can be enumerated as follows:
- From within
cv_wait(9) and associated
methods when the current LWP voluntarily relinquishes the CPU to wait for
some resource to become available.
- From within
preempt(9) when the current
LWP voluntarily relinquishes the CPU or when the kernel prepares a return
to user-mode execution.
- In the signal handling code if a signal is delivered that
causes an LWP to stop (see
issignal(9)).
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 (see
getrlimit(2)). Exceeding the
soft limit results in a
SIGXCPU
signal to be posted to
the LWP, while exceeding the hard limit will cause a
SIGKILL
.
Unless
l->l_switchto is not
NULL
,
mi_switch() will call
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,
mi_switch() will
hand over control to the machine-dependent function
cpu_switchto(9) to switch
to the new LWP.
mi_switch() has to be called with the LWP lock held (through
calling
lwp_lock() first) and at the
splsched(9) interrupt
protection level. It returns with the LWP lock released.
RETURN VALUES
mi_switch() returns 1 if a context switch was performed to a
different LWP, 0 otherwise.
SEE ALSO
condvar(9),
cpu_switchto(9),
csf(9),
pmap(9),
ras(9),
sched_4bsd(9),
splsched(9)