进程调度API之should_resched

static __always_inline bool should_resched(int preempt_offset)用于判断当前函数能否被抢占.其使用的例子如下：本例中中通过should_resched的返回值resched 来决定当前thread 是否可以被调度int __cond_resched_lock(spinlock_t *lock){int resched = should_resched(PREEMPT_LOCK_OFFSET);int ret = 0;lockdep_assert_held(lock);if (spin_needbreak(lock) || resched) {spin_unlock(lock);if (resched)preempt_schedule_common();elsecpu_relax();ret = 1;spin_lock(lock);}return ret;}其源码分析如下：static __always_inline bool should_resched(int preempt_offset){return unlikely(preempt_count() == preempt_offset &&tif_need_resched());}这个函数的返回值被unlikely修饰，说明这个函数大部分情况下返回的是false这个函数的返回值由两部分组成，分别是preempt_count() == preempt_offset 和 tif_need_resched()其中tif_need_resched() 这个函数之前已经分析过了，主要看当前thread中是否置位TIF_NEED_RESCHED所以重点看一下preempt_count() == preempt_offset 条件static __always_inline int preempt_count(void){return READ_ONCE(current_thread_info()->preempt_count);}preempt_count 函数其实就是读一下当前thread的preempt_count。最终在should_resched 中判断是否等于形参。这里的形参其实也是一个固定值#if defined(CONFIG_PREEMPT_COUNT)# define PREEMPT_DISABLE_OFFSETPREEMPT_OFFSET#else# define PREEMPT_DISABLE_OFFSET0#endif/* * The preempt_count offset after spin_lock() */#define PREEMPT_LOCK_OFFSETPREEMPT_DISABLE_OFFSET可以看到如果没有定义CONFIG_PREEMPT_COUNT的话，其实就是0一般情况下我们也不会定义这个宏所以should_resched的作用和preempt_count_dec_and_test 是一样的