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The following is the code for the process scheduling function and its related functions.

Void schedule (void)

{

Int i,next,c

Struct task_struct * * p

/ * check alarm, wake up any interruptible tasks that have got a signal * /

/ / signal and status of the processing process

For (p = & LAST_TASK; p > & FIRST_TASK;-- p)

If (* p) {

/ *

The value of alarm is set by calling the alarm function. See the alarm function. The process can call the alarm function to set a time.

Then the alram signal, alarm, will be triggered when it expires.

Alarm & & (* p)-> alarm

Signal | = (1signal & ~ _ BLOCKABLE & (* p)-> blocked) & &

(* p)-> state==TASK_INTERRUPTIBLE)

(* p)-> state=TASK_RUNNING

}

/ * this is the scheduler proper: * /

/ / start scheduling and select the appropriate process for execution

While (1) {

C =-1

Next = 0

I = NR_TASKS

P = & task [NR _ TASKS]

While (--I) {

If (! *-p)

Continue

/ / find out the process with the largest time slice, indicating that it has the shortest execution time.

If ((* p)-> state = = TASK_RUNNING & & (* p)-> counter > c)

C = (* p)-> counter, next = I

}

/ / there are still processes to be executed, c greater than or equal to 0

If (c) break

/ / No break indicates that c equals 0, that is, all process time slices have been executed and need to be reset.

For (p = & LAST_TASK; p > & FIRST_TASK;-- p)

If (* p)

/ / the higher the priority, the longer the execution time, the greater the chance of being selected for execution

(* p)-> counter = ((* p)-> counter > > 1) +

(* p)-> priority

}

/ / switch process

Switch_to (next)

}

# define switch_to (n) {\

Struct {long a dint b;} _ _ tmp;\

/ / ecx is the pcb header address corresponding to the nth process. To determine whether the next process to be switched is the currently executed process, there is no need to switch.

_ _ asm__ ("cmpl% ecx,_current\ n\ t"\

"je 1f\ n\ t"\

/ / copy the tss selector of the nth process to _ _ tmp.b

"movw% dx,%1\ n\ t"\

/ / update the current variable so that the execution ecx,ecx of the current variable points to task [n]

"xchgl% ecx,_current\ n\ t"\

/ / ljmp and a tss selector to achieve process switching

"ljmp% 0\ n\ t"\

/ / ignore

"cmpl% ecx,_last_task_used_math\ n\ t"\

"jne 1f\ n\ t"\

"clts\ n"\

"1:"\

: "m" (* & _ _ tmp.a), "m" (* & _ _ tmp.b),\

"d" (_ TSS (n)), "c" ((long) task [n]));\

}

Int sys_alarm (long seconds)

{

Int old = current- > alarm

If (old)

Old = (old-jiffies) / HZ

/ / 1 second equals 100 jiffies

Current- > alarm = (seconds > 0)? (jiffies+HZ*seconds): 0

Return (old)

}

/ / modify the priority of process execution. The larger the increment, the lower the priority if the conditions are met.

Int sys_nice (long increment)

{

If (current- > priority-increment > 0)

Current- > priority-= increment

Return 0

}

Each process has an execution time, and each clock interrupt reduces one unit of time. If the time runs out, reschedule directly, otherwise the process can continue to execute. When the process is scheduled, the system will select the longest process to prevent some processes from not being executed, and recalculate the time when the time slices of all processes are consumed.

Do_timer function

If ((--current- > counter) > 0) return

Current- > counter=0

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