进程调度API之preempt

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进程调度API之preempt

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); else mdelay(longdelay_ms / 10); #ifdef CONFIG_PREEMPT if (!(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。

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