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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, 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-han-
		dling code after detection of a	change in the signal disposi-
		tion 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	run-
     ning 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 ex-
     ceeding 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 as-
     sociated 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 ac-
     tual 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


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