mi_switch(9) [netbsd man page]

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

NAME

     mi_switch -- machine independent context switch prelude

SYNOPSIS

     int
     mi_switch(struct lwp *l);

DESCRIPTION

     The mi_switch() function implements the machine-independent prelude to an LWP 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 three major uses of mi_switch() can be
     enumerated as follows:

	   1.	From within cv_wait(9) and associated methods when the current LWP voluntarily relinquishes the CPU to wait for some resource to
		become available.

	   2.	From within preempt(9) when the current LWP voluntarily relinquishes the CPU or when the kernel prepares a return to user-mode
		execution.

	   3.	In the signal handling code if a signal is delivered that causes an LWP to stop (see issignal(9)).

     mi_switch() records the amount of time the current LWP has been running in the LWP structure and checks this value against the CPU time lim-
     its allocated to the LWP (see getrlimit(2)).  Exceeding the soft limit results in a SIGXCPU signal to be posted to the LWP, while exceeding
     the hard limit will cause a SIGKILL.

     Unless l->l_switchto is not NULL, mi_switch() will call sched_nextlwp() to select a new LWP from the scheduler's runqueue structures.  If no
     runnable LWP is found, the idle LWP is used.  If the new LWP is not equal to the current one, mi_switch() will hand over control to the
     machine-dependent function cpu_switchto(9) to switch to the new LWP.

     mi_switch() has to be called with the LWP lock held (through calling lwp_lock() first) and at the splsched(9) interrupt protection level.	It
     returns with the LWP lock released.

RETURN VALUES

     mi_switch() returns 1 if a context switch was performed to a different LWP, 0 otherwise.

SEE ALSO

     condvar(9), cpu_switchto(9), csf(9), pmap(9), ras(9), sched_4bsd(9), splsched(9)

BSD
								   July 21, 2007							       BSD

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

	   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 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)).  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.

SEE ALSO

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

BSD
								 November 24, 1996							       BSD