switch to another thread context
() function implements the
machine independent prelude to a thread 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 various major uses of
can be enumerated as follows:
- From within a function such as
when the current thread voluntarily relinquishes the CPU to wait for some
resource or lock to become available.
- 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 dependent 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
- In the signal handling code (see
if a signal is delivered that causes a process to stop.
- When a thread dies in
and control of the processor can be passed to the next runnable
where a thread needs to stop execution due to the suspension state of the
process as a whole.
() 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
Exceeding the soft limit results in a
signal to be posted to the process,
while exceeding the hard limit will cause a
If the thread is still in the
() 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 enabled, the associated
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,
() hands over control to the
machine dependent routine
which will perform the actual thread context switch.
() first saves the context of the
current thread. Next, it calls
() 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.
() is similar to
() 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
() when the kernel terminates
the current thread and switches into a new thread.
To protect the
all of these functions must be called with the