trusty (9) cpu_throw.9freebsd.gz

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(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 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)