NAME    [Toc]    [Back]

     mi_switch, cpu_switch - switch to another process context

SYNOPSIS    [Toc]    [Back]

     void
     mi_switch(void);

     void
     cpu_switch(void);

DESCRIPTION    [Toc]    [Back]

     The mi_switch() function implements the machine-independent prelude to a
     process 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 sleep(9) and tsleep(9) when the current process
                voluntarily relinquishes the CPU to wait for some resource to
                become available.

           2.   After handling a trap (e.g. a system call or 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 process
                might be available to run.  The latter event is communicated
                by the machine-independent scheduling routines by calling the
                machine-dependent need_resched(void).

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

     mi_switch() records the amount of time the current process has been running
 in the process structure 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.  After these administrative
 tasks are done, mi_switch() hands over control to the machine dependent
 routine cpu_switch(void), which will perform the actual process context
 switch.

     cpu_switch() will make a choice amongst the processes which are ready to
     run from a priority queue data-structure.  The priority queue consists of
     an array qs[NQS] of queue header structures each of which identifies a
     list of runnable processes of equal priority (see <sys/proc.h>).  A single
 word whichqs containing a bit mask identifying non-empty queues
     assists in selecting a process quickly.  cpu_switch() must remove the
     first process from the list on the queue with the highest priority (lower
     indices in qs indicate higher priority), and assign the address of its
     process structure to the global variable curproc.  If no processes are
     available on the run queues, cpu_switch() shall go into an ``idle'' loop.
     The idle loop must allow interrupts to be taken that will eventually
     cause processes to appear again on the run queues.  The variable curproc
     should be NULL while cpu_switch() waits for this to happen.

     Note that mi_switch() and thus cpu_switch() should be called at
     splhigh(9).

SEE ALSO    [Toc]    [Back]

     tsleep(9), wakeup(9)

BSD                            November 24, 1996                           BSD