Linux系统内核抢占补丁的基本原理

　　while (current->need_resched) {

　　ctx_sw_off();

　　current->state |= TASK_PREEMPTED;

　　schedule();

　　current->state &= ~TASK_PREEMPTED;

　　ctx_sw_on_no_preempt();

　　}

　　}

　　#endif

　　asmlinkage void schedule(void)

　　{

　　struct schedule_data * sched_data;

　　struct task_struct *prev, *next, *p;

　　struct list_head *tmp;

　　int this_cpu, c;

　　#ifdef CONFIG_PREEMPT

　　ctx_sw_off();

　　#endif

　　if (!current->active_mm) BUG();

　　need_resched_back:

　　prev = current;

　　this_cpu = prev->processor;

　　if (in_interrupt())

　　goto scheduling_in_interrupt;

　　release_kernel_lock(prev, this_cpu);

　　/* Do "administrative" work here while we don't hold any locks */

　　if (softirq_active(this_cpu) &softirq_mask(this_cpu))

　　goto handle_softirq;

　　handle_softirq_back:

　　/*

　　* 'sched_data' is protected by the fact that we can run

　　* only one process per CPU.

　　*/

　　sched_data = &aligned_data[this_cpu].schedule_data;

　　spin_lock_irq(&runqueue_lock);

　　/* move an exhausted RR process to be last.. */

　　if (prev->policy == SCHED_RR)

　　goto move_rr_last;

　　move_rr_back:

　　switch (prev->state) {

　　case TASK_INTERRUPTIBLE:

　　if (signal_pending(prev)) {

　　prev->state = TASK_RUNNING;

　　break;

　　}

　　default:

　　#ifdef CONFIG_PREEMPT

　　if (prev->state &TASK_PREEMPTED)

　　break; 如果是内核抢占调度,则保留运行队列

　　#endif

　　del_from_runqueue(prev);

　　#ifdef CONFIG_PREEMPT

　　case TASK_PREEMPTED:

　　#endif

　　case TASK_RUNNING:

　　}

　　prev->need_resched = 0;

　　/*

　　* this is the scheduler proper:

　　*/

　　repeat_schedule:

　　/*

　　* Default process to select..

　　*/

　　next = idle_task(this_cpu);

　　c = -1000;

　　if (task_on_runqueue(prev))

　　goto still_running;

　　still_running_back:

　　list_for_each(tmp, &runqueue_head) {

　　p = list_entry(tmp, struct task_struct, run_list);

　　if (can_schedule(p, this_cpu)) {

　　int weight = goodness(p, this_cpu, prev->active_mm);

　　if (weight >c)

　　c = weight, next = p;

　　}

　　}

　　/* Do we need to re-calculate counters? */

　　if (!c)

　　goto recalculate;

　　/*

　　* from this point on nothing can prevent us from

　　* switching to the next task, save this fact in

　　* sched_data.

　　*/

　　sched_data->curr = next;

　　#ifdef CONFIG_SMP

　　next->has_cpu = 1;

　　next->processor = this_cpu;

　　#endif

　　spin_unlock_irq(&runqueue_lock);

　　if (prev == next)

　　goto same_process;

　　#ifdef CONFIG_SMP

　　/*

　　* maintain the per-process 'last schedule' value.

　　* (this has to be recalculated even if we reschedule to

　　* the same process) Currently this is only used on SMP,

　　* and it's approximate, so we do not have to maintain

　　* it while holding the runqueue spinlock.

　　*/

　　sched_data->last_schedule = get_cycles();

　　/*

　　* We drop the scheduler lock early (it's a global spinlock),

　　* thus we have to lock the previous process from getting

　　* rescheduled during switch_to().

　　*/

　　#endif /* CONFIG_SMP */

　　kstat.context_swtch++;

　　/*

　　* there are 3 processes which are affected by a context switch:

　　*

　　* prev == .... ==> (last => next)

　　*

　　* It's the 'much more previous' 'prev' that is on next's stack,

　　* but prev is set to (the just run) 'last' process by switch_to().

　　* This might sound slightly confusing but makes tons of sense.

　　*/

　　prepare_to_switch();

　　{

　　struct mm_struct *mm = next->mm;

　　struct mm_struct *oldmm = prev->active_mm;

　　if (!mm) {

　　if (next->active_mm) BUG();

　　next->active_mm = oldmm;

　　atomic_inc(&oldmm->mm_count);

　　enter_lazy_tlb(oldmm, next, this_cpu);

　　} else {

　　if (next->active_mm != mm) BUG();

　　switch_mm(oldmm, mm, next, this_cpu);

　　}

　　if (!prev->mm) {

　　prev->active_mm = NULL;

　　mmdrop(oldmm);

　　}

　　}

　　/*

　　* This just switches the register state and the

　　* stack.

　　*/

　　switch_to(prev, next, prev);

　　__schedule_tail(prev);

　　same_process:

　　reacquire_kernel_lock(current);

　　if (current->need_resched)

　　goto need_resched_back;

　　#ifdef CONFIG_PREEMPT

　　ctx_sw_on_no_preempt();

　　#endif

　　return;

　　recalculate:

　　{

　　struct task_struct *p;

　　spin_unlock_irq(&runqueue_lock);

　　read_lock(&tasklist_lock);

　　for_each_task(p)

　　p->counter = (p->counter >>1) + NICE_TO_TICKS(p->nice);

　　read_unlock(&tasklist_lock);

　　spin_lock_irq(&runqueue_lock);

　　}

　　goto repeat_schedule;

　　still_running:

　　c = goodness(prev, this_cpu, prev->active_mm);

　　next = prev;

　　goto still_running_back;

　　handle_softirq:

　　do_softirq();

　　goto handle_softirq_back;

　　move_rr_last:

　　if (!prev->counter) {

　　prev->counter = NICE_TO_TICKS(prev->nice);

　　move_last_runqueue(prev);

　　}

　　goto move_rr_back;

　　scheduling_in_interrupt:

　　printk("Scheduling in interrupt\n");

　　BUG();

　　return;

　　}

　　void schedule_tail(struct task_struct *prev)

　　{

　　__schedule_tail(prev);

　　#ifdef CONFIG_PREEMPT

　　ctx_sw_on();

　　#endif

　　}