进程调度API之preempt_schedule

asmlinkage __visible void __sched notrace preempt_schedule(void)这个函数用于检查当前thread是否可以主动让出cpu给其他thread 运行其使用的例子如下：static void torture_rwsem_write_delay(struct torture_random_state *trsp){const unsigned long longdelay_ms = 100;/* We want a long delay occasionally to force massive contention.  */if (!(torture_random(trsp) %      (cxt.nrealwriters_stress * 2000 * longdelay_ms)))mdelay(longdelay_ms * 10);elsemdelay(longdelay_ms / 10);#ifdef CONFIG_PREEMPTif (!(torture_random(trsp) % (cxt.nrealwriters_stress * 20000)))preempt_schedule();  /* Allow test to be preempted. */#endif}这个例子中就通过preempt_schedule 来判断当前thread是否可以主动释放cpu其源码分析如下：asmlinkage __visible void __sched notrace preempt_schedule(void){/* * If there is a non-zero preempt_count or interrupts are disabled, * we do not want to preempt the current task. Just return.. */if (likely(!preemptible()))return;preempt_schedule_common();}首先这里判断当前thread是否可以让出cpu的条件是preemptible。#define preemptible()(preempt_count() == 0 && !irqs_disabled())这个条件就是当前thrad 的preempt_count等于0，且没有禁止irq不过这里用likely修饰！，所以这个条件一般会成立，也就是当前thread 不会主动让出cpu假定条件成立，则调用preempt_schedule_common 来开始调度static void __sched notrace preempt_schedule_common(void){do {/* * Because the function tracer can trace preempt_count_sub() * and it also uses preempt_enable/disable_notrace(), if * NEED_RESCHED is set, the preempt_enable_notrace() called * by the function tracer will call this function again and * cause infinite recursion. * * Preemption must be disabled here before the function * tracer can trace. Break up preempt_disable() into two * calls. One to disable preemption without fear of being * traced. The other to still record the preemption latency, * which can also be traced by the function tracer. */preempt_disable_notrace();preempt_latency_start(1);__schedule(true);preempt_latency_stop(1);preempt_enable_no_resched_notrace();/* * Check again in case we missed a preemption opportunity * between schedule and now. */} while (need_resched());}这个函数的核心就是调用__schedule 来让出cpu。