NAME
mi_switch,
cpu_switchto —
switch to another process
context
SYNOPSIS
#include
<sys/param.h>
#include <sys/proc.h>
void
mi_switch(void);
void
cpu_switchto(struct
proc *old, struct proc
*new);
DESCRIPTION
The
mi_switch()
function implements the machine-independent prelude to a process 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 functions like
tsleep(9), when the current process sleeps to wait for some
resource to become available, or from within functions like
yield(), when it relinquishes the CPU to allow other processes to make progress. - After handling a trap (e.g., a system call or device
interrupt) when the kernel prepares a return to user-mode execution. This
case is typically handled by machine-dependent trap-handling code after
detection of a change in the signal disposition of the current process, or
when a higher priority process might be available to run. The latter event
is communicated by the machine-independent scheduling routines by calling
the machine-dependent
need_resched(void). - In the signal handling code (see sys/kern/kern_sig.c) if a signal is delivered that causes a process to stop.
mi_switch()
records the amount of time the current process has been running in the
process structure and checks this value against the CPU time limits
allocated to the process (see
getrlimit(2)). Exceeding the soft limit results in a
SIGXCPU signal to be posted to the process, while
exceeding the hard limit will cause a SIGKILL. After
these administrative tasks are done, mi_switch()
chooses the next process to run and hands over control to the machine
dependent routine cpu_switchto(), which will perform
the actual process context switch.
cpu_switchto()
will save the context of the old process and switch to the new one. A
special case is when the old process is NULL which
means that the old process has exited and doesn't need to be saved.
Note that
mi_switch()
and thus cpu_switchto() must be called while holding
the scheduler lock.