CPU在内核中运行时并不是处处不可抢占的，内核中存在一些空隙，在这时进行抢占是安全的，内核抢占补丁的基本原理就是将SMP可并行的代码段看成是可以进行内核抢占的区域。

　　2.4内核正好细化了多CPU下的内核线程同步机构，对不可并行的指令块用spinlock和rwlock作了细致的表示，该补丁的实现可谓水到渠成。具体的方法就是在进程的任务结构上增加一个preempt_count变量作为内核抢占锁，它随着spinlock和rwlock一起加锁和解锁。当preempt_count为0时表示可以进行内核调度。内核调度器的入口为preempt_schedule()，它将当前进程标记为TASK_PREEMPTED状态再调用schedule()，在TASK_PREEMPTED状态，schedule()不会将进程从运行队列中删除。

　　下面是内核抢占补丁的主要代码示意：

　　arch/i386/kernel/entry.S:

　　preempt_count = 4 # 将task_struct中的flags用作preempt_count,flags被移到了别

　　的位置

　　ret_from_exception: # 从异常返回

　　#ifdef CONFIG_SMP

　　GET_CURRENT(%ebx)

　　movl processor(%ebx),%eax

　　shll $CONFIG_X86_L1_CACHE_SHIFT,%eax

　　movl SYMBOL_NAME(irq_stat)(,%eax),%ecx # softirq_active

　　testl SYMBOL_NAME(irq_stat)+4(,%eax),%ecx # softirq_mask

　　#else

　　movl SYMBOL_NAME(irq_stat),%ecx # softirq_active

　　testl SYMBOL_NAME(irq_stat)+4,%ecx # softirq_mask

　　#endif

　　jne handle_softirq

　　#ifdef CONFIG_PREEMPT

　　cli

　　incl preempt_count(%ebx) # 异常的入口没有禁止内核调度的指令,与ret_from_intr

　　匹配一下

　　#endif

　　ENTRY(ret_from_intr) # 硬件中断的返回

　　GET_CURRENT(%ebx)

　　#ifdef CONFIG_PREEMPT

　　cli

　　decl preempt_count(%ebx) # 恢复内核抢占标志

　　#endif

　　movl EFLAGS(%esp),%eax # mix EFLAGS and CS

　　movb CS(%esp),%al

　　testl $(VM_MASK | 3),%eax # return to VM86 mode or non-supervisor?

　　jne ret_with_reschedule

　　#ifdef CONFIG_PREEMPT

　　cmpl $0,preempt_count(%ebx)

　　jnz restore_all # 如果preempt_count非零则表示禁止内核抢占

　　cmpl $0,need_resched(%ebx)

　　jz restore_all #

　　movl SYMBOL_NAME(irq_stat)+irq_stat_local_bh_count CPU_INDX,%ecx

　　addl SYMBOL_NAME(irq_stat)+irq_stat_local_irq_count CPU_INDX,%ecx

　　jnz restore_all

　　incl preempt_count(%ebx)

　　sti

　　call SYMBOL_NAME(preempt_schedule)

　　jmp ret_from_intr # 新进程返回,返回ret_from_intr恢复抢占标志后再返回

　　#else

　　jmp restore_all

　　#endif

　　ALIGN

　　handle_softirq:

　　#ifdef CONFIG_PREEMPT

　　cli

　　GET_CURRENT(%ebx)

　　incl preempt_count(%ebx)

　　sti

　　#endif

　　call SYMBOL_NAME(do_softirq)

　　jmp ret_from_intr

　　ALIGN

　　reschedule:

　　call SYMBOL_NAME(schedule) # test

　　jmp ret_from_sys_call

　　include/asm/hw_irq.h:

　　...

　　#ifdef CONFIG_PREEMPT

　　#define BUMP_CONTEX_SWITCH_LOCK \

　　GET_CURRENT \

　　"incl 4(%ebx)\n\t"

　　#else

　　#define BUMP_CONTEX_SWITCH_LOCK

　　#endif

　　#define SAVE_ALL \ 硬件中断保护入口现场

　　"cld\n\t" \

　　"pushl %es\n\t" \

　　"pushl %ds\n\t" \

　　"pushl %eax\n\t" \

　　"pushl %ebp\n\t" \

　　"pushl %edi\n\t" \

　　"pushl %esi\n\t" \

　　"pushl %edx\n\t" \

　　"pushl %ecx\n\t" \

　　"pushl %ebx\n\t" \

　　"movl $" STR(__KERNEL_DS) ",%edx\n\t" \

　　"movl %edx,%ds\n\t" \

　　"movl %edx,%es\n\t" \

　　BUMP_CONTEX_SWITCH_LOCK # 硬件中断的入口禁止内核抢占

　　include/linux/spinlock.h:

　　#ifdef CONFIG_PREEMPT

　　#define switch_lock_count() current->preempt_count

　　#define in_ctx_sw_off() (switch_lock_count().counter) 判断当前进程的抢占计数

　　是否非零

　　#define atomic_ptr_in_ctx_sw_off() (&switch_lock_count())

　　#define ctx_sw_off() \ 禁止内核抢占

　　do { \

　　atomic_inc(atomic_ptr_in_ctx_sw_off()); \ 当前进程的内核抢占计数增1

　　} while (0)

　　#define ctx_sw_on_no_preempt() \ 允许内核抢占

　　do { \

　　atomic_dec(atomic_ptr_in_ctx_sw_off()); \ 当前进程的内核抢占计数减1

　　} while (0)

　　#define ctx_sw_on() \ 允许并完成内核抢占

　　do { \

　　if (atomic_dec_and_test(atomic_ptr_in_ctx_sw_off()) &&\

　　current->need_resched) \

　　preempt_schedule(); \

　　} while (0)

　　#define spin_lock(lock) \

　　do { \

　　ctx_sw_off(); \ 进入自旋锁时禁止抢占

　　_raw_spin_lock(lock); \

　　} while(0)

　　#define spin_trylock(lock) ({ctx_sw_off(); _raw_spin_trylock(lock) ? \锁定并

　　测试原来是否上锁

　　1 : ({ctx_sw_on(); 0;});})

　　#define spin_unlock(lock) \

　　do { \

　　_raw_spin_unlock(lock); \

　　ctx_sw_on(); \ 离开自旋锁时允许并完成内核抢占

　　} while (0)

　　#define read_lock(lock) ({ctx_sw_off(); _raw_read_lock(lock);})

　　#define read_unlock(lock) ({_raw_read_unlock(lock); ctx_sw_on();})

　　#define write_lock(lock) ({ctx_sw_off(); _raw_write_lock(lock);})

　　#define write_unlock(lock) ({_raw_write_unlock(lock); ctx_sw_on();})

　　#define write_trylock(lock) ({ctx_sw_off(); _raw_write_trylock(lock) ? \

　　1 : ({ctx_sw_on(); 0;});})

　　...

　　include/asm/softirq.h:

　　#define cpu_bh_disable(cpu) do { ctx_sw_off(); local_bh_count(cpu)++; barrie

　　r(); } while (0)

　　#define cpu_bh_enable(cpu) do { barrier(); local_bh_count(cpu)--;ctx_sw_on()

　　; } while (0)

　　kernel/schedule.c:

　　#ifdef CONFIG_PREEMPT

　　asmlinkage void preempt_schedule(void)

　　{

　　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

　　}