|MI_SWITCH(9)||Kernel Developer's Manual||MI_SWITCH(9)|
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_switchcan be enumerated as follows:
- 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.
- 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
- In the signal handling code (see issignal(9)) if a signal is delivered that causes a process to stop.
- When a thread dies in thread_exit(9) and control of the processor can be passed to the next runnable thread.
- 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
If the thread is still in the
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
cpu_switch(), which will perform
the actual thread context switch.
cpu_switch() first saves the context of
the current thread. Next, it calls
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)
|November 24, 1996||Linux 4.9.0-9-amd64|