NAME
mi_switch —
machine independent context switch
prelude
SYNOPSIS
int
mi_switch(struct
lwp *l);
DESCRIPTION
Themi_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)