进程调度API之should_resched
2017-12-12 08:55
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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(); else cpu_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_OFFSET PREEMPT_OFFSET #else # define PREEMPT_DISABLE_OFFSET 0 #endif /* * The preempt_count offset after spin_lock() */ #define PREEMPT_LOCK_OFFSET PREEMPT_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
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