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<li class="toctree-l2"><a class="reference internal" href="../chapter4/3sv39-implementation-1.html">实现 SV39 多级页表机制（上）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/2fs-implementation-1.html">简易文件系统 easy-fs (上)</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter7/1pipe.html">管道</a></li>
<li class="toctree-l2"><a class="reference internal" href="../chapter7/2cmdargs-and-redirection.html">命令行参数与标准 I/O 重定向</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter8/1thread-kernel.html">内核态的线程管理</a></li>
<li class="toctree-l2"><a class="reference internal" href="../chapter8/2lock.html">锁机制</a></li>
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<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-a/index.html">附录 A：Rust 系统编程资料</a><input class="toctree-checkbox" id="toctree-checkbox-17" name="toctree-checkbox-17" role="switch" type="checkbox"/><label for="toctree-checkbox-17"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
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<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
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<h1>管理多道程序<a class="headerlink" href="#id1" title="永久链接至标题"></a></h1>
<p>而内核为了管理任务，需要维护任务信息，相关内容包括：</p>
<ul class="simple">
<li><p>任务运行状态：未初始化、准备执行、正在执行、已退出</p></li>
<li><p>任务控制块：维护任务状态和任务上下文</p></li>
<li><p>任务相关系统调用：程序主动暂停 <code class="docutils literal notranslate"><span class="pre">sys_yield</span></code> 和主动退出 <code class="docutils literal notranslate"><span class="pre">sys_exit</span></code></p></li>
</ul>
<div class="section" id="yield">
<h2>yield 系统调用<a class="headerlink" href="#yield" title="永久链接至标题"></a></h2>
<img alt="../_images/multiprogramming.png" src="../_images/multiprogramming.png"/>
<p>上图描述了一种多道程序执行的典型情况。其中横轴为时间线，纵轴为正在执行的实体。
开始时，蓝色应用向外设提交了一个请求，外设随即开始工作，
但是它要一段时间后才能返回结果。蓝色应用于是调用 <code class="docutils literal notranslate"><span class="pre">sys_yield</span></code> 交出 CPU 使用权，
内核让绿色应用继续执行。一段时间后 CPU 切换回蓝色应用，发现外设仍未返回结果，
于是再次 <code class="docutils literal notranslate"><span class="pre">sys_yield</span></code> 。直到第二次切换回蓝色应用，外设才处理完请求，于是蓝色应用终于可以向下执行了。</p>
<p>我们还会遇到很多其他需要等待其完成才能继续向下执行的事件，调用 <code class="docutils literal notranslate"><span class="pre">sys_yield</span></code> 可以避免等待过程造成的资源浪费。</p>
<div class="literal-block-wrapper docutils container" id="id5">
<div class="code-block-caption"><span class="caption-text">第三章新增系统调用（一）</span><a class="headerlink" href="#id5" title="永久链接至代码"></a></div>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="sd">/// 功能：应用主动交出 CPU 所有权并切换到其他应用。</span>
<span class="sd">/// 返回值：总是返回 0。</span>
<span class="sd">/// syscall ID：124</span>
<span class="k">fn</span> <span class="nf">sys_yield</span><span class="p">()</span><span class="w"> </span>-&gt; <span class="kt">isize</span><span class="p">;</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>用户库对应的实现和封装：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// user/src/syscall.rs</span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">sys_yield</span><span class="p">()</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">syscall</span><span class="p">(</span><span class="n">SYSCALL_YIELD</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">])</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="c1">// user/src/lib.rs</span>
<span class="c1">// yield 是 Rust 的关键字</span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">yield_</span><span class="p">()</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"> </span><span class="n">sys_yield</span><span class="p">()</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>下文介绍内核应如何实现该系统调用。</p>
</div>
<div class="section" id="id2">
<h2>任务控制块与任务运行状态<a class="headerlink" href="#id2" title="永久链接至标题"></a></h2>
<p>任务运行状态暂包括如下几种：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="c1">// os/src/task/task.rs</span>
<span class="linenos">2</span>
<span class="linenos">3</span><span class="cp">#[derive(Copy, Clone, PartialEq)]</span><span class="w"></span>
<span class="linenos">4</span><span class="k">pub</span><span class="w"> </span><span class="k">enum</span> <span class="nc">TaskStatus</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">5</span><span class="w"> </span><span class="n">UnInit</span><span class="p">,</span><span class="w"> </span><span class="c1">// 未初始化</span>
<span class="linenos">6</span><span class="w"> </span><span class="n">Ready</span><span class="p">,</span><span class="w"> </span><span class="c1">// 准备运行</span>
<span class="linenos">7</span><span class="w"> </span><span class="n">Running</span><span class="p">,</span><span class="w"> </span><span class="c1">// 正在运行</span>
<span class="linenos">8</span><span class="w"> </span><span class="n">Exited</span><span class="p">,</span><span class="w"> </span><span class="c1">// 已退出</span>
<span class="linenos">9</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>任务状态外和任务上下文一并保存在名为 <strong>任务控制块</strong> (Task Control Block) 的数据结构中：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="c1">// os/src/task/task.rs</span>
<span class="linenos">2</span>
<span class="linenos">3</span><span class="cp">#[derive(Copy, Clone)]</span><span class="w"></span>
<span class="linenos">4</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">TaskControlBlock</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">5</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">task_status</span>: <span class="nc">TaskStatus</span><span class="p">,</span><span class="w"></span>
<span class="linenos">6</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">task_cx</span>: <span class="nc">TaskContext</span><span class="p">,</span><span class="w"></span>
<span class="linenos">7</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>任务控制块非常重要。在内核中，它就是应用的管理单位。后面的章节我们还会不断向里面添加更多内容。</p>
</div>
<div class="section" id="id3">
<h2>任务管理器<a class="headerlink" href="#id3" title="永久链接至标题"></a></h2>
<p>内核需要一个全局的任务管理器来管理这些任务控制块：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/task/mod.rs</span>
<span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">TaskManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">num_app</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">inner</span>: <span class="nc">UPSafeCell</span><span class="o">&lt;</span><span class="n">TaskManagerInner</span><span class="o">&gt;</span><span class="p">,</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="k">struct</span> <span class="nc">TaskManagerInner</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">tasks</span>: <span class="p">[</span><span class="n">TaskControlBlock</span><span class="p">;</span><span class="w"> </span><span class="n">MAX_APP_NUM</span><span class="p">],</span><span class="w"></span>
<span class="w"> </span><span class="n">current_task</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>这里用到了变量与常量分离的编程风格：字段 <code class="docutils literal notranslate"><span class="pre">num_app</span></code> 表示应用数目，它在 <code class="docutils literal notranslate"><span class="pre">TaskManager</span></code> 初始化后将保持不变；
而包裹在 <code class="docutils literal notranslate"><span class="pre">TaskManagerInner</span></code> 内的任务控制块数组 <code class="docutils literal notranslate"><span class="pre">tasks</span></code>，以及正在执行的应用编号 <code class="docutils literal notranslate"><span class="pre">current_task</span></code> 会在执行过程中变化。</p>
<p>初始化 <code class="docutils literal notranslate"><span class="pre">TaskManager</span></code> 的全局实例 <code class="docutils literal notranslate"><span class="pre">TASK_MANAGER</span></code>：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1">// os/src/task/mod.rs</span>
<span class="linenos"> 2</span>
<span class="linenos"> 3</span><span class="n">lazy_static</span><span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 4</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">ref</span><span class="w"> </span><span class="n">TASK_MANAGER</span>: <span class="nc">TaskManager</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 5</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">num_app</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get_num_app</span><span class="p">();</span><span class="w"></span>
<span class="linenos"> 6</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">tasks</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span><span class="n">TaskControlBlock</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 7</span><span class="w"> </span><span class="n">task_cx</span>: <span class="nc">TaskContext</span>::<span class="n">zero_init</span><span class="p">(),</span><span class="w"></span>
<span class="linenos"> 8</span><span class="w"> </span><span class="n">task_status</span>: <span class="nc">TaskStatus</span>::<span class="n">UnInit</span><span class="p">,</span><span class="w"></span>
<span class="linenos"> 9</span><span class="w"> </span><span class="p">};</span><span class="w"> </span><span class="n">MAX_APP_NUM</span><span class="p">];</span><span class="w"></span>
<span class="linenos">10</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">tasks</span><span class="p">.</span><span class="n">iter_mut</span><span class="p">().</span><span class="n">enumerate</span><span class="p">().</span><span class="n">take</span><span class="p">(</span><span class="n">num_app</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">11</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">task_cx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskContext</span>::<span class="n">goto_restore</span><span class="p">(</span><span class="n">init_app_cx</span><span class="p">(</span><span class="n">i</span><span class="p">));</span><span class="w"></span>
<span class="linenos">12</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">task_status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Ready</span><span class="p">;</span><span class="w"></span>
<span class="linenos">13</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">14</span><span class="w"> </span><span class="n">TaskManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">15</span><span class="w"> </span><span class="n">num_app</span><span class="p">,</span><span class="w"></span>
<span class="linenos">16</span><span class="w"> </span><span class="n">inner</span>: <span class="nc">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">17</span><span class="w"> </span><span class="n">UPSafeCell</span>::<span class="n">new</span><span class="p">(</span><span class="n">TaskManagerInner</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">18</span><span class="w"> </span><span class="n">tasks</span><span class="p">,</span><span class="w"></span>
<span class="linenos">19</span><span class="w"> </span><span class="n">current_task</span>: <span class="mi">0</span><span class="p">,</span><span class="w"></span>
<span class="linenos">20</span><span class="w"> </span><span class="p">})</span><span class="w"></span>
<span class="linenos">21</span><span class="w"> </span><span class="p">},</span><span class="w"></span>
<span class="linenos">22</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">23</span><span class="w"> </span><span class="p">};</span><span class="w"></span>
<span class="linenos">24</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<ul class="simple">
<li><p>第 5 行：调用 <code class="docutils literal notranslate"><span class="pre">loader</span></code> 子模块提供的 <code class="docutils literal notranslate"><span class="pre">get_num_app</span></code> 接口获取链接到内核的应用总数；</p></li>
<li><p>第 10~12 行：依次对每个任务控制块进行初始化，将其运行状态设置为 <code class="docutils literal notranslate"><span class="pre">Ready</span></code> ，并在它的内核栈栈顶压入一些初始化
上下文，然后更新它的 <code class="docutils literal notranslate"><span class="pre">task_cx</span></code> 。一些细节我们会稍后介绍。</p></li>
<li><p>从第 14 行开始：创建 <code class="docutils literal notranslate"><span class="pre">TaskManager</span></code> 实例并返回。</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p>关于 Rust 迭代器语法如 <code class="docutils literal notranslate"><span class="pre">iter_mut/(a..b)</span></code> ，及其方法如 <code class="docutils literal notranslate"><span class="pre">enumerate/map/find/take</span></code>，请参考 Rust 官方文档。</p>
</div>
</div>
<div class="section" id="sys-yield-sys-exit">
<h2>实现 sys_yield 和 sys_exit<a class="headerlink" href="#sys-yield-sys-exit" title="永久链接至标题"></a></h2>
<p><code class="docutils literal notranslate"><span class="pre">sys_yield</span></code> 的实现用到了 <code class="docutils literal notranslate"><span class="pre">task</span></code> 子模块提供的 <code class="docutils literal notranslate"><span class="pre">suspend_current_and_run_next</span></code> 接口，这个接口如字面含义，就是暂停当前的应用并切换到下个应用。</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/syscall/process.rs</span>
<span class="k">use</span><span class="w"> </span><span class="k">crate</span>::<span class="n">task</span>::<span class="n">suspend_current_and_run_next</span><span class="p">;</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">sys_yield</span><span class="p">()</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">suspend_current_and_run_next</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="mi">0</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">sys_exit</span></code> 基于 <code class="docutils literal notranslate"><span class="pre">task</span></code> 子模块提供的 <code class="docutils literal notranslate"><span class="pre">exit_current_and_run_next</span></code> 接口，它的含义是退出当前的应用并切换到下个应用：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/syscall/process.rs</span>
<span class="k">use</span><span class="w"> </span><span class="k">crate</span>::<span class="n">task</span>::<span class="n">exit_current_and_run_next</span><span class="p">;</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">sys_exit</span><span class="p">(</span><span class="n">exit_code</span>: <span class="kt">i32</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="fm">println!</span><span class="p">(</span><span class="s">"[kernel] Application exited with code {}"</span><span class="p">,</span><span class="w"> </span><span class="n">exit_code</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">exit_current_and_run_next</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="fm">panic!</span><span class="p">(</span><span class="s">"Unreachable in sys_exit!"</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>那么 <code class="docutils literal notranslate"><span class="pre">suspend_current_and_run_next</span></code> 和 <code class="docutils literal notranslate"><span class="pre">exit_current_and_run_next</span></code> 各是如何实现的呢？</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/task/mod.rs</span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">suspend_current_and_run_next</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">TASK_MANAGER</span><span class="p">.</span><span class="n">mark_current_suspended</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="n">TASK_MANAGER</span><span class="p">.</span><span class="n">run_next_task</span><span class="p">();</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">exit_current_and_run_next</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">TASK_MANAGER</span><span class="p">.</span><span class="n">mark_current_exited</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="n">TASK_MANAGER</span><span class="p">.</span><span class="n">run_next_task</span><span class="p">();</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>它们都是先修改当前应用的运行状态，然后尝试切换到下一个应用。修改运行状态比较简单，实现如下：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="c1">// os/src/task/mod.rs</span>
<span class="linenos">2</span>
<span class="linenos">3</span><span class="k">impl</span><span class="w"> </span><span class="n">TaskManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">4</span><span class="w"> </span><span class="k">fn</span> <span class="nf">mark_current_suspended</span><span class="p">(</span><span class="o">&amp;</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">5</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
<span class="linenos">6</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">current</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">current_task</span><span class="p">;</span><span class="w"></span>
<span class="linenos">7</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="n">current</span><span class="p">].</span><span class="n">task_status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Ready</span><span class="p">;</span><span class="w"></span>
<span class="linenos">8</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">9</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>以 <code class="docutils literal notranslate"><span class="pre">mark_current_suspended</span></code> 为例。首先获得里层 <code class="docutils literal notranslate"><span class="pre">TaskManagerInner</span></code> 的可变引用，然后修改任务控制块数组 <code class="docutils literal notranslate"><span class="pre">tasks</span></code> 中当前任务的状态。</p>
<p>再看 <code class="docutils literal notranslate"><span class="pre">run_next_task</span></code> 的实现：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1">// os/src/task/mod.rs</span>
<span class="linenos"> 2</span>
<span class="linenos"> 3</span><span class="k">impl</span><span class="w"> </span><span class="n">TaskManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 4</span><span class="w"> </span><span class="k">fn</span> <span class="nf">run_next_task</span><span class="p">(</span><span class="o">&amp;</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 5</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nb">Some</span><span class="p">(</span><span class="n">next</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">find_next_task</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 6</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
<span class="linenos"> 7</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">current</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">current_task</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 8</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="n">next</span><span class="p">].</span><span class="n">task_status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Running</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 9</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">current_task</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">next</span><span class="p">;</span><span class="w"></span>
<span class="linenos">10</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">current_task_cx_ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&amp;</span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="n">current</span><span class="p">].</span><span class="n">task_cx</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">mut</span><span class="w"> </span><span class="n">TaskContext</span><span class="p">;</span><span class="w"></span>
<span class="linenos">11</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">next_task_cx_ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&amp;</span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="n">next</span><span class="p">].</span><span class="n">task_cx</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">const</span><span class="w"> </span><span class="n">TaskContext</span><span class="p">;</span><span class="w"></span>
<span class="linenos">12</span><span class="w"> </span><span class="nb">drop</span><span class="p">(</span><span class="n">inner</span><span class="p">);</span><span class="w"></span>
<span class="linenos">13</span><span class="w"> </span><span class="c1">// before this, we should drop local variables that must be dropped manually</span>
<span class="linenos">14</span><span class="w"> </span><span class="k">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">15</span><span class="w"> </span><span class="n">__switch</span><span class="p">(</span><span class="n">current_task_cx_ptr</span><span class="p">,</span><span class="w"> </span><span class="n">next_task_cx_ptr</span><span class="p">);</span><span class="w"></span>
<span class="linenos">16</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">17</span><span class="w"> </span><span class="c1">// go back to user mode</span>
<span class="linenos">18</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">19</span><span class="w"> </span><span class="fm">panic!</span><span class="p">(</span><span class="s">"All applications completed!"</span><span class="p">);</span><span class="w"></span>
<span class="linenos">20</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">21</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">22</span>
<span class="linenos">23</span><span class="w"> </span><span class="k">fn</span> <span class="nf">find_next_task</span><span class="p">(</span><span class="o">&amp;</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="nb">Option</span><span class="o">&lt;</span><span class="kt">usize</span><span class="o">&gt;</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">24</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
<span class="linenos">25</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">current</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">current_task</span><span class="p">;</span><span class="w"></span>
<span class="linenos">26</span><span class="w"> </span><span class="p">(</span><span class="n">current</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="o">..</span><span class="n">current</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">num_app</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"></span>
<span class="linenos">27</span><span class="w"> </span><span class="p">.</span><span class="n">map</span><span class="p">(</span><span class="o">|</span><span class="n">id</span><span class="o">|</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">num_app</span><span class="p">)</span><span class="w"></span>
<span class="linenos">28</span><span class="w"> </span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="o">|</span><span class="n">id</span><span class="o">|</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="o">*</span><span class="n">id</span><span class="p">].</span><span class="n">task_status</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Ready</span><span class="p">)</span><span class="w"></span>
<span class="linenos">29</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">30</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">run_next_task</span></code> 会调用 <code class="docutils literal notranslate"><span class="pre">find_next_task</span></code> 方法尝试寻找一个运行状态为 <code class="docutils literal notranslate"><span class="pre">Ready</span></code> 的应用并获得其 ID 。
如果找不到， 说明所有应用都执行完了， <code class="docutils literal notranslate"><span class="pre">find_next_task</span></code> 将返回 <code class="docutils literal notranslate"><span class="pre">None</span></code> ，内核 panic 退出。
如果能够找到下一个可运行应用，我们就调用 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 切换任务。</p>
<p>切换任务之前，我们要手动 drop 掉我们获取到的 <code class="docutils literal notranslate"><span class="pre">TaskManagerInner</span></code> 可变引用。
因为函数还没有返回， <code class="docutils literal notranslate"><span class="pre">inner</span></code> 不会自动销毁。我们只有令 <code class="docutils literal notranslate"><span class="pre">TASK_MANAGER</span></code> 的 <code class="docutils literal notranslate"><span class="pre">inner</span></code> 字段回到未被借用的状态，下次任务切换时才能再借用。</p>
<p>我们可以总结一下应用的运行状态变化图：</p>
<img alt="../_images/fsm-coop.png" src="../_images/fsm-coop.png"/>
</div>
<div class="section" id="id4">
<h2>第一次进入用户态<a class="headerlink" href="#id4" title="永久链接至标题"></a></h2>
<p>我们在第二章中介绍过 CPU 第一次从内核态进入用户态的方法，只需在内核栈上压入构造好的 Trap 上下文，
然后 <code class="docutils literal notranslate"><span class="pre">__restore</span></code> 即可。本章要在此基础上做一些扩展。</p>
<p>在初始化任务控制块时，我们是这样做的：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/task/mod.rs</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">tasks</span><span class="p">.</span><span class="n">iter_mut</span><span class="p">().</span><span class="n">enumerate</span><span class="p">().</span><span class="n">take</span><span class="p">(</span><span class="n">num_app</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">task_cx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskContext</span>::<span class="n">goto_restore</span><span class="p">(</span><span class="n">init_app_cx</span><span class="p">(</span><span class="n">i</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">task_status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Ready</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">init_app_cx</span></code> 在 <code class="docutils literal notranslate"><span class="pre">loader</span></code> 子模块中定义，它向内核栈压入了一个 Trap 上下文，并返回压入 Trap 上下文后 <code class="docutils literal notranslate"><span class="pre">sp</span></code> 的值。
这个 Trap 上下文的构造方式与第二章相同。</p>
<p><code class="docutils literal notranslate"><span class="pre">goto_restore</span></code> 保存传入的 <code class="docutils literal notranslate"><span class="pre">sp</span></code>，并将 <code class="docutils literal notranslate"><span class="pre">ra</span></code> 设置为 <code class="docutils literal notranslate"><span class="pre">__restore</span></code> 的入口地址，构造任务上下文后返回。这样，任务管理器中各个应用的任务上下文就得到了初始化。</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/task/context.rs</span>
<span class="k">impl</span><span class="w"> </span><span class="n">TaskContext</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">goto_restore</span><span class="p">(</span><span class="n">kstack_ptr</span>: <span class="kt">usize</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">extern</span><span class="w"> </span><span class="s">"C"</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">fn</span> <span class="nf">__restore</span><span class="p">();</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="bp">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">ra</span>: <span class="nc">__restore</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">sp</span>: <span class="nc">kstack_ptr</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">s</span>: <span class="p">[</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="mi">12</span><span class="p">],</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>在 <code class="docutils literal notranslate"><span class="pre">rust_main</span></code> 中我们调用 <code class="docutils literal notranslate"><span class="pre">task::run_first_task</span></code> 来执行第一个应用：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1">// os/src/task/mod.rs</span>
<span class="linenos"> 2</span>
<span class="linenos"> 3</span><span class="k">fn</span> <span class="nf">run_first_task</span><span class="p">(</span><span class="o">&amp;</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 4</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
<span class="linenos"> 5</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">task0</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&amp;</span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">tasks</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span><span class="w"></span>
<span class="linenos"> 6</span><span class="w"> </span><span class="n">task0</span><span class="p">.</span><span class="n">task_status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskStatus</span>::<span class="n">Running</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 7</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">next_task_cx_ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&amp;</span><span class="n">task0</span><span class="p">.</span><span class="n">task_cx</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">const</span><span class="w"> </span><span class="n">TaskContext</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 8</span><span class="w"> </span><span class="nb">drop</span><span class="p">(</span><span class="n">inner</span><span class="p">);</span><span class="w"></span>
<span class="linenos"> 9</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">_unused</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TaskContext</span>::<span class="n">zero_init</span><span class="p">();</span><span class="w"></span>
<span class="linenos">10</span><span class="w"> </span><span class="c1">// before this, we should drop local variables that must be dropped manually</span>
<span class="linenos">11</span><span class="w"> </span><span class="k">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">12</span><span class="w"> </span><span class="n">__switch</span><span class="p">(</span><span class="o">&amp;</span><span class="k">mut</span><span class="w"> </span><span class="n">_unused</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">mut</span><span class="w"> </span><span class="n">TaskContext</span><span class="p">,</span><span class="w"> </span><span class="n">next_task_cx_ptr</span><span class="p">);</span><span class="w"></span>
<span class="linenos">13</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">14</span><span class="w"> </span><span class="fm">panic!</span><span class="p">(</span><span class="s">"unreachable in run_first_task!"</span><span class="p">);</span><span class="w"></span>
<span class="linenos">15</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>我们显式声明了一个 <code class="docutils literal notranslate"><span class="pre">_unused</span></code> 变量，并将它的地址作为第一个参数传给 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> ，
声明此变量的意义仅仅是为了避免其他数据被覆盖。</p>
<p>在 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 中恢复 <code class="docutils literal notranslate"><span class="pre">sp</span></code> 后， <code class="docutils literal notranslate"><span class="pre">sp</span></code> 将指向 <code class="docutils literal notranslate"><span class="pre">init_app_cx</span></code> 构造的 Trap 上下文，后面就回到第二章的情况了。
此外， <code class="docutils literal notranslate"><span class="pre">__restore</span></code> 的实现需要做出变化：它 <strong>不再需要</strong> 在开头 <code class="docutils literal notranslate"><span class="pre">mv</span> <span class="pre">sp,</span> <span class="pre">a0</span></code> 了。因为在 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 之后，<code class="docutils literal notranslate"><span class="pre">sp</span></code> 就已经正确指向了我们需要的 Trap 上下文地址。</p>
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<li><a class="reference internal" href="#">管理多道程序</a><ul>
<li><a class="reference internal" href="#yield">yield 系统调用</a></li>
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