mi_switch(9)

NAME

mi_switch, cpu_switch, cpu_throw - switch to another thread

context

SYNOPSIS

#include <sys/param.h>
#include <sys/proc.h>
void
mi_switch(void);
void
cpu_switch(void);
void
cpu_throw(void);

DESCRIPTION

The mi_switch() function implements the machine independent

prelude to a
thread context switch. It is called from only a few distin

guished places
in the kernel code as a result of the principle of non-pre

emptable kernel
mode execution. The various major uses of mi_switch can be

enumerated as
follows:

1. From within sleep(9), tsleep(9) and msleep(9)

when the current
thread voluntarily relinquishes the CPU to wait

for some
resource to become available.

2. After handling a trap (e.g. a system call, device

interrupt)
when the kernel prepares a return to user-mode

execution.
This case is typically handled by machine depen

dent trap-handling code after detection of a change in the

signal disposition of the current process, or when a higher

priority thread
might be available to run. The latter event is

communicated
by the machine independent scheduling routines by

calling the
machine defined need_resched().

3. In the signal handling code (see issignal(9)) if

a signal is
delivered that causes a process to stop.

4. When a thread dies in thread_exit(9) and control

of the pro
cessor can be passed to the next runnable thread.

5. In thread_suspend_check(9) where a thread needs

to stop execu
tion due to the suspension state of the process

as a whole.

mi_switch() records the amount of time the current thread

has been running in the process structures and checks this value against

the CPU time
limits allocated to the process (see getrlimit(2)). Exceed

ing the soft
limit results in a SIGXCPU signal to be posted to the pro

cess, while
exceeding the hard limit will cause a SIGKILL.

If the thread is still in the TDS_RUNNING state, mi_switch()

will put it
back onto the run queue, assuming that it will want to run

again soon.
If it is in one of the other states and KSE threading is en

abled, the
associated KSE will be made available to any higher priority

threads from
the same group, to allow them to be scheduled next.

After these administrative tasks are done, mi_switch() hands

over control
to the machine dependent routine cpu_switch(), which will

perform the
actual thread context switch.

cpu_switch() first saves the context of the current thread.

Next, it
calls choosethread() to determine which thread to run next.

Finally, it
reads in the saved context of the new thread and starts to

execute the
new thread.

cpu_throw() is similar to cpu_switch() except that it does

not save the
context of the old thread. This function is useful when the

kernel does
not have an old thread context to save, such as when CPUs

other than the
boot CPU perform their first task switch, or when the kernel

does not
care about the state of the old thread, such as in

thread_exit() when the
kernel terminates the current thread and switches into a new

thread.

To protect the runqueue(9), all of these functions must be

called with
the sched_lock mutex held.

SEE ALSO

issignal(9), mutex(9), runqueue(9), tsleep(9), wakeup(9)

BSD November 24, 1996

BSD