Manual Pages  — MI_SWITCH

1. From within a function such as sleepq_wait(9) or turnstile_wait() when the current thread voluntarily relinquishes the CPU to wait for some resource or lock to become available.
2. Involuntary preemption due to arrival of a higher-priority thread.
3. At the tail end of critical_exit(9), if preemption was deferred due to the critical section.
4. Within the TDA_SCHED AST handler, when rescheduling before the return to usermode was requested. There are several reasons for this, a notable one coming from sched_clock() when the running thread has exceeded its time slice.
5. In the signal handling code (see issignal(9)) if a signal is delivered that causes a process to stop.
6. In thread_suspend_check() where a thread needs to stop execution due to the suspension state of the process as a whole.
7. In kern_yield(9) when a thread wants to voluntarily relinquish the processor.

The flags argument to mi_switch() indicates the context switch type. One of the following must be passed:

SWT_OWEPREEMPT	Switch due to delayed preemption after exiting a critical section.
SWT_TURNSTILE	Switch after propagating scheduling priority to the owner of a resource.
SWT_SLEEPQ	Begin waiting on a sleepqueue(9).
SWT_RELINQUISH	Yield call.
SWT_NEEDRESCHED	Rescheduling was requested.
SWT_IDLE	Switch from the idle thread.
SWT_IWAIT	A kernel thread which handles interrupts has finished work and must wait for interrupts to schedule additional work.
SWT_SUSPEND	Thread suspended.
SWT_REMOTEPREEMPT	Preemption by a higher-priority thread, initiated by a remote processor.
SWT_REMOTEWAKEIDLE	Idle thread preempted, initiated by a remote processor.
SWT_BIND	The running thread has been bound to another processor and must be switched out.

In addition to the switch type, callers must specify the nature of the switch by performing a bitwise OR with one of the SW_VOL or SW_INVOL flags, but not both. Respectively, these flags denote whether the context switch is voluntary or involuntary on the part of the current thread. For an involuntary context switch in which the running thread is being preempted, the caller should also pass the SW_PREEMPT flag.

Upon entry to mi_switch(), the current thread must be holding its assigned thread lock. It may be unlocked as part of the context switch. After they have been rescheduled and execution resumes, threads will exit mi_switch() with their thread lock unlocked.

mi_switch() records the amount of time the current thread has been running before handing control over to the scheduler, via sched_switch(). After selecting a new thread to run, the scheduler will call cpu_switch() to perform the low-level context switch.

cpu_switch() is the machine-dependent function that performs the actual switch from the running thread oldtd to the chosen thread newtd.