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<li class="toctree-l2 current current-page"><a class="current reference internal" href="#">任务切换</a></li>
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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="../chapter4/4sv39-implementation-2.html">实现 SV39 多级页表机制（下）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/1process.html">与进程有关的重要系统调用</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/2core-data-structures.html">进程管理的核心数据结构</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="../chapter6/2fs-implementation-2.html">简易文件系统 easy-fs (下)</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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<p class="caption" role="heading"><span class="caption-text">开发注记</span></p>
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<div class="section" id="id1">
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<h1>任务切换<a class="headerlink" href="#id1" title="永久链接至标题">¶</a></h1>
|
||
<p>本节我们将见识操作系统的核心机制—— <strong>任务切换</strong> ，
|
||
即应用在运行中主动或被动地交出 CPU 的使用权，内核可以选择另一个程序继续执行。
|
||
内核需要保证用户程序两次运行期间，任务上下文（如寄存器、栈等）保持一致。</p>
|
||
<div class="section" id="id2">
|
||
<h2>任务切换的设计与实现<a class="headerlink" href="#id2" title="永久链接至标题">¶</a></h2>
|
||
<p>任务切换与上一章提及的 Trap 控制流切换相比，有如下异同：</p>
|
||
<ul class="simple">
|
||
<li><p>与 Trap 切换不同，它不涉及特权级切换，部分由编译器完成；</p></li>
|
||
<li><p>与 Trap 切换相同，它对应用是透明的。</p></li>
|
||
</ul>
|
||
<p>事实上，任务切换是来自两个不同应用在内核中的 Trap 控制流之间的切换。
|
||
当一个应用 Trap 到 S 态 OS 内核中进行进一步处理时，
|
||
其 Trap 控制流可以调用一个特殊的 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 函数。
|
||
在 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 返回之后，Trap 控制流将继续从调用该函数的位置继续向下执行。
|
||
而在调用 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 之后到返回前的这段时间里，
|
||
原 Trap 控制流 <code class="docutils literal notranslate"><span class="pre">A</span></code> 会先被暂停并被切换出去， CPU 转而运行另一个应用的 Trap 控制流 <code class="docutils literal notranslate"><span class="pre">B</span></code> 。
|
||
<code class="docutils literal notranslate"><span class="pre">__switch</span></code> 返回之后，原 Trap 控制流 <code class="docutils literal notranslate"><span class="pre">A</span></code> 才会从某一条 Trap 控制流 <code class="docutils literal notranslate"><span class="pre">C</span></code> 切换回来继续执行。</p>
|
||
<p>我们需要在 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 中保存 CPU 的某些寄存器，它们就是 <strong>任务上下文</strong> (Task Context)。</p>
|
||
<p>下面我们给出 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 的实现：</p>
|
||
<div class="highlight-riscv notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c"># os/src/task/switch.S</span>
|
||
<span class="linenos"> 2</span>
|
||
<span class="linenos"> 3</span><span class="nf">.altmacro</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="nf">.macro</span><span class="w"> </span><span class="nb">SAVE_SN</span><span class="w"> </span><span class="ni">n</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="w"> </span><span class="nd">sd</span><span class="w"> </span><span class="no">s</span>\<span class="ni">n</span>,<span class="w"> </span>(\<span class="ni">n</span>+<span class="m">2</span>)*<span class="m">8</span>(<span class="na">a0</span>)<span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="nf">.endm</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="nf">.macro</span><span class="w"> </span><span class="nb">LOAD_SN</span><span class="w"> </span><span class="ni">n</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="w"> </span><span class="nd">ld</span><span class="w"> </span><span class="no">s</span>\<span class="ni">n</span>,<span class="w"> </span>(\<span class="ni">n</span>+<span class="m">2</span>)*<span class="m">8</span>(<span class="na">a1</span>)<span class="w"></span>
|
||
<span class="linenos"> 9</span><span class="nf">.endm</span><span class="w"></span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="nf">.section</span><span class="w"> </span><span class="nf">.text</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="nf">.globl</span><span class="w"> </span><span class="nb">__switch</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="nb">__switch:</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="c"># __switch(</span>
|
||
<span class="linenos">14</span><span class="w"> </span><span class="c"># current_task_cx_ptr: *mut TaskContext,</span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="c"># next_task_cx_ptr: *const TaskContext</span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="c"># )</span>
|
||
<span class="linenos">17</span><span class="w"> </span><span class="c"># save kernel stack of current task</span>
|
||
<span class="linenos">18</span><span class="w"> </span><span class="nd">sd</span><span class="w"> </span><span class="na">sp</span>,<span class="w"> </span><span class="m">8</span>(<span class="na">a0</span>)<span class="w"></span>
|
||
<span class="linenos">19</span><span class="w"> </span><span class="c"># save ra & s0~s11 of current execution</span>
|
||
<span class="linenos">20</span><span class="w"> </span><span class="nd">sd</span><span class="w"> </span><span class="na">ra</span>,<span class="w"> </span><span class="m">0</span>(<span class="na">a0</span>)<span class="w"></span>
|
||
<span class="linenos">21</span><span class="w"> </span><span class="nf">.set</span><span class="w"> </span><span class="ni">n</span>,<span class="w"> </span><span class="m">0</span><span class="w"></span>
|
||
<span class="linenos">22</span><span class="w"> </span><span class="nf">.rept</span><span class="w"> </span><span class="m">12</span><span class="w"></span>
|
||
<span class="linenos">23</span><span class="w"> </span><span class="nb">SAVE_SN</span><span class="w"> </span>%<span class="ni">n</span><span class="w"></span>
|
||
<span class="linenos">24</span><span class="w"> </span><span class="nf">.set</span><span class="w"> </span><span class="ni">n</span>,<span class="w"> </span><span class="ni">n</span><span class="w"> </span>+<span class="w"> </span><span class="m">1</span><span class="w"></span>
|
||
<span class="linenos">25</span><span class="w"> </span><span class="nf">.endr</span><span class="w"></span>
|
||
<span class="linenos">26</span><span class="w"> </span><span class="c"># restore ra & s0~s11 of next execution</span>
|
||
<span class="linenos">27</span><span class="w"> </span><span class="nd">ld</span><span class="w"> </span><span class="na">ra</span>,<span class="w"> </span><span class="m">0</span>(<span class="na">a1</span>)<span class="w"></span>
|
||
<span class="linenos">28</span><span class="w"> </span><span class="nf">.set</span><span class="w"> </span><span class="ni">n</span>,<span class="w"> </span><span class="m">0</span><span class="w"></span>
|
||
<span class="linenos">29</span><span class="w"> </span><span class="nf">.rept</span><span class="w"> </span><span class="m">12</span><span class="w"></span>
|
||
<span class="linenos">30</span><span class="w"> </span><span class="nb">LOAD_SN</span><span class="w"> </span>%<span class="ni">n</span><span class="w"></span>
|
||
<span class="linenos">31</span><span class="w"> </span><span class="nf">.set</span><span class="w"> </span><span class="ni">n</span>,<span class="w"> </span><span class="ni">n</span><span class="w"> </span>+<span class="w"> </span><span class="m">1</span><span class="w"></span>
|
||
<span class="linenos">32</span><span class="w"> </span><span class="nf">.endr</span><span class="w"></span>
|
||
<span class="linenos">33</span><span class="w"> </span><span class="c"># restore kernel stack of next task</span>
|
||
<span class="linenos">34</span><span class="w"> </span><span class="nd">ld</span><span class="w"> </span><span class="na">sp</span>,<span class="w"> </span><span class="m">8</span>(<span class="na">a1</span>)<span class="w"></span>
|
||
<span class="linenos">35</span><span class="w"> </span><span class="nd">ret</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>它的两个参数分别是当前和即将被切换到的 Trap 控制流的 <code class="docutils literal notranslate"><span class="pre">task_cx_ptr</span></code> ，从 RISC-V 调用规范可知，它们分别通过寄存器 <code class="docutils literal notranslate"><span class="pre">a0/a1</span></code> 传入。</p>
|
||
<p>内核先把 <code class="docutils literal notranslate"><span class="pre">current_task_cx_ptr</span></code> 中包含的寄存器值逐个保存，再把 <code class="docutils literal notranslate"><span class="pre">next_task_cx_ptr</span></code> 中包含的寄存器值逐个恢复。</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">TaskContext</span></code> 里包含的寄存器有：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="c1">// os/src/task/context.rs</span>
|
||
<span class="linenos">2</span><span class="cp">#[repr(C)]</span><span class="w"></span>
|
||
<span class="linenos">3</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">TaskContext</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">4</span><span class="w"> </span><span class="n">ra</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos">5</span><span class="w"> </span><span class="n">sp</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos">6</span><span class="w"> </span><span class="n">s</span>: <span class="p">[</span><span class="kt">usize</span><span class="p">;</span><span class="w"> </span><span class="mi">12</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><code class="docutils literal notranslate"><span class="pre">s0~s11</span></code> 是被调用者保存寄存器， <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 是用汇编编写的，编译器不会帮我们处理这些寄存器。
|
||
保存 <code class="docutils literal notranslate"><span class="pre">ra</span></code> 很重要，它记录了 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 函数返回之后应该跳转到哪里继续执行。</p>
|
||
<p>我们将这段汇编代码 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 解释为一个 Rust 函数：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="c1">// os/src/task/switch.rs</span>
|
||
<span class="linenos">2</span>
|
||
<span class="linenos">3</span><span class="n">core</span>::<span class="n">arch</span>::<span class="fm">global_asm!</span><span class="p">(</span><span class="fm">include_str!</span><span class="p">(</span><span class="s">"switch.S"</span><span class="p">));</span><span class="w"></span>
|
||
<span class="linenos">4</span>
|
||
<span class="linenos">5</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="linenos">6</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">__switch</span><span class="p">(</span><span class="w"></span>
|
||
<span class="linenos">7</span><span class="w"> </span><span class="n">current_task_cx_ptr</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">8</span><span class="w"> </span><span class="n">next_task_cx_ptr</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">9</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>我们会调用该函数来完成切换功能，而不是直接跳转到符号 <code class="docutils literal notranslate"><span class="pre">__switch</span></code> 的地址。
|
||
因此在调用前后，编译器会帮我们保存和恢复调用者保存寄存器。</p>
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