mi_switch, cpu_switch - switch to another process context
void
mi_switch(void);
void
cpu_switch(void);
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).
tsleep(9), wakeup(9)
BSD November 24, 1996 BSD
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