MI_SWITCH(9)             BSD Kernel Developer's Manual            MI_SWITCH(9)

     mi_switch, cpu_switch, cpu_throw — switch to another thread context

     #include <sys/param.h>
     #include <sys/proc.h>




     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(9), 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
     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.

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

BSD                            November 24, 1996                           BSD