详解Linux内核进程调度函数schedule()的触发和执行时机
<p>内核的调度操作分为触发和执行两个部分，触发时仅仅设置一下当前进程的TIF_NEED_RESCHED标志，执行的时候则是通过schedule()函数来完成进程的选择和切换。当前进程的thread_info->flags中TIF_NEED_RESCHED位表示需要调用schedule()函数进行调度。内核在两种情况下会设置该标志，一个是在时钟中断进行周期性的检查时，另一个是在被唤醒进程的优先级比正在运行的进程的优先级高时。</p>
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<strong>周期性地更新当前任务的状态时：</strong></p>
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定时中断处理函数中会调用schedule_tick()用于处理关于调度的周期性检查和处理，其调用路径是和时钟处理有关的tick_periodic()->update_process_times()->scheduler_tick()或者tick_sched_handle()->update_process_times()->scheduler_tick()，主要用于更新就绪队列的时钟、CPU负载和当前任务的运行时间统计等，如下所示：</p>
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<code class="cpp comments">//linux-3.13/kernel/sched/core.c</code>
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<code class="cpp keyword bold">void</code> <code class="cpp plain">scheduler_tick(</code><code class="cpp keyword bold">void</code><code class="cpp plain">)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">cpu = smp_processor_id(); </code><code class="cpp comments">//获取当前cpu编号</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">rq *rq = cpu_rq(cpu); </code><code class="cpp comments">//取得对应cpu的rq（就绪队列）</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">task_struct *curr = rq->curr; </code><code class="cpp comments">//获取当前运行的任务</code>
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<code class="cpp spaces"> </code><code class="cpp plain">sched_clock_tick();</code>
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<code class="cpp spaces"> </code><code class="cpp plain">raw_spin_lock(&rq->lock);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">update_rq_clock(rq); </code><code class="cpp comments">//更新队列时钟</code>
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<code class="cpp spaces"> </code><code class="cpp plain">curr->sched_class->task_tick(rq, curr, 0); </code><code class="cpp comments">//调用当前任务的调度类对应的函数</code>
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<code class="cpp spaces"> </code><code class="cpp plain">update_cpu_load_active(rq); </code><code class="cpp comments">//更新本处理器的负载</code>
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<code class="cpp spaces"> </code><code class="cpp plain">raw_spin_unlock(&rq->lock);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">perf_event_task_tick();</code>
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<code class="cpp preprocessor">#ifdef CONFIG_SMP</code>
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<code class="cpp spaces"> </code><code class="cpp plain">rq->idle_balance = idle_cpu(cpu);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">trigger_load_balance(rq, cpu); </code><code class="cpp comments">//必要时进行负载均衡</code>
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<code class="cpp preprocessor">#endif</code>
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<code class="cpp spaces"> </code><code class="cpp plain">rq_last_tick_reset(rq);</code>
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<code class="cpp plain">}</code>
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<p>
其中curr->sched_class->task_tick(rq, curr, 0);这行代码调用了当前任务的调度类的task_tick()函数，这个函数根据具体情况决定是否需要对当前任务设置TIF_NEED_RESCHED标志，如果需要则最终调用set_tsk_need_resched()设置该标志。需要注意的是，此处仅仅是设置标志而没有执行schedule()函数，在各种系统调用、中断的返回代码最后，才会根据这个标志来决定是否执行schedule()函数。</p>
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<strong>睡眠的任务被唤醒时：</strong></p>
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当睡眠任务所等待的事件到达时，内核（例如驱动程序的中断处理函数）将会调用wake_up()唤醒相关的任务，并最终调用try_to_wake_up()。它完成三件事：将任务重新添加到就绪队列，将运行标志设置为TASK_RUNNING，如果被唤醒的任务可以抢占当前运行任务则设置当前任务的TIF_NEED_RESCHED标志。</p>
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设置了TIF_NEED_RESCHED标志之后，真正调用执行schedule()函数的时机只有两种，第一种是系统调用或者中断返回时，根据TIF_NEED_RESCHED标志决定是否调用schedule()函数（从效率方面考虑，趁着还在内核态把该处理的事情处理完毕）；第二种情况是当前任务因为原因需要睡眠，进程睡眠后立即调用schedule()函数，在内核中这种情况也比较多，比如磁盘、网卡等设备驱动程序中。</p>
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<strong>参考文献：《Linux技术内幕》 </strong></p>
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PS：刚开始学习Linux内核的时候很容易被各种结构体各种概念充斥脑海，一团乱麻。这时候需要把它们各自负责的功能以及之间相互的配合理清楚，推荐这本书。看完《Linux内核设计与实现》后可以相互比照，效果不错。</p>
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以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。</p>
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原文链接：http://blog.csdn.net/Q_AN1314/article/details/79133046</p>
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