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mi_switch, cpu_switch, cpu_throw — switch to another thread context
The mi_switch() 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 mi_switch can be enumerated as
1. From within a function such as cv_wait(9), mtx_lock, or
tsleep(9) when the current thread voluntarily relinquishes the
CPU to wait for some resource or lock 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 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 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
processor can be passed to the next runnable thread.
5. In thread_suspend_check(9) where a thread needs to stop
execution due to the suspension state of the process as a
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)). Exceeding the
soft limit results in a SIGXCPU signal to be posted to the process, 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 enabled, 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
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.
cv_wait(9), issignal(9), mutex(9), runqueue(9), tsleep(9), wakeup(9)