MI_SWITCH(9) OpenBSD Kernel Manual MI_SWITCH(9)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:
1. From within tsleep(9) when the current process voluntarily
relinquishes the CPU to wait for some resource to become
available.
2. 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).
3. In the signal handling code (see issignal(9)) 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. For a process which
accumulated longer than 10 minutes of CPU time, its nice level is raised
to 4. 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() should be called at
splhigh(9).
SEE ALSOspl(9), tsleep(9), wakeup(9)OpenBSD 4.9 June 26, 2008 OpenBSD 4.9
