CTXSW(9)

NAME
mi_switch, cpu_switch - switch to another process context

SYNOPSIS
void
mi_switch(void);
void
cpu_switch(void);

DESCRIPTION
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 sq 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
tsleep(9), wakeup(9)