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Linux 进程中 Stop, Park, Freeze - 魅族内核团队
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<h1>Linux 进程中 Stop, Park, Freeze</h1>
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Posted by Peng Weilin on
2016-07-11
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<p>在调试内核的时候，经常会碰到几个相近的概念：进程 stop、进程 park、进程 freeze。这几个名词看起来都是停止进程，那么他们之间的区别和应用场景在分别是什么呢？下面就来分析一番。</p>
<blockquote>
<p>本文的代码分析基于 Linux kernel 3.18.22，最好的学习方法还是 “RTFSC”</p>
</blockquote>
<h2 id="1-进程-stop"><a href="#1-进程-stop" class="headerlink" title="1. 进程 stop"></a>1. 进程 stop</h2><p>进程 stop 分成两种：用户进程 stop 和内核进程 stop。</p>
<p>用户进程 stop 可以通过给进程发送 STOP 信号来实现，可以参考“Linux Signal”这一篇的描述。但是对内核进程来说不会响应信号，如果碰到需要 stop 内核进程的场景怎么处理？比如：我们在设备打开的时候创建了内核处理进程，在设备关闭的时候需要 stop 内核进程。</p>
<p>Linux 实现了一套 <code>kthread_stop()</code> 的机制来实现内核进程 stop。</p>
<h3 id="1-1-内核进程的创建"><a href="#1-1-内核进程的创建" class="headerlink" title="1.1 内核进程的创建"></a>1.1 内核进程的创建</h3><p>内核进程创建过程，是理解本篇的基础。</p>
<p>可以看到 <code>kthread_create()</code> 并不是自己去创建内核进程，而是把创建任务推送给 <code>kthreadd()</code> 进程执行。</p>
<p><code>kthreadd()</code> -> <code>create_kthread()</code> -> <code>kernel_thread()</code> 创建的新进程也不是直接使用用户的函数 <code>threadfn()</code>，而是创建通用函数 <code>kthread()</code>，<code>kthread()</code> 再来调用 <code>threadfn()</code>。</p>
<ul>
<li>kernel/kthread.c:</li>
</ul>
<p><img src="stop_kthread_create.png" alt="kthread_create"></p>
<h3 id="1-2-内核进程的-stop"><a href="#1-2-内核进程的-stop" class="headerlink" title="1.2 内核进程的 stop"></a>1.2 内核进程的 stop</h3><p>如果内核进程需要支持 <code>kthread_stop()</code>，需要根据以下框架来写代码。用户在主循环中调用 <code>kthread_should_stop()</code> 来判断当前 kthread 是否需要 stop，如果被 stop 则退出循环。</p>
<p>这种代码为什么不做到通用代码 <code>kthread()</code> 中？这应该是和 Linux 的设计思想相关的。Linux 运行内核态的策略比较灵活，而对用户态的策略更加严格统一。</p>
<p><img src="stop_kthread_should_stop.png" alt="kthread_should_stop"></p>
<p><code>kthread_should_stop()</code> 和 <code>kthread_stop()</code> 的代码实现：</p>
<ul>
<li>kernel/kthread.c:</li>
<li><code>kthread_should_stop()</code>/<code>kthread_stop()</code></li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="function"><span class="type">bool</span> <span class="title">kthread_should_stop</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="comment">// (1) 判断进程所在 kthread 结构中的 KTHREAD_SHOULD_STOP 是否被置位</span></span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">test_bit</span>(KTHREAD_SHOULD_STOP, &<span class="built_in">to_kthread</span>(current)->flags);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">kthread_stop</span><span class="params">(<span class="keyword">struct</span> task_struct *k)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">kthread</span> *kthread;</span><br><span class="line"> <span class="type">int</span> ret;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">trace_sched_kthread_stop</span>(k);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">get_task_struct</span>(k);</span><br><span class="line"> kthread = <span class="built_in">to_live_kthread</span>(k);</span><br><span class="line"> <span class="keyword">if</span> (kthread) {</span><br><span class="line"> <span class="comment">// (2) 置位进程所在 kthread 结构中的 KTHREAD_SHOULD_STOP</span></span><br><span class="line"> <span class="built_in">set_bit</span>(KTHREAD_SHOULD_STOP, &kthread->flags);</span><br><span class="line"> <span class="comment">// (3) unpark & wake_up 进程来响应 stop 信号</span></span><br><span class="line"> __kthread_unpark(k, kthread);</span><br><span class="line"> <span class="built_in">wake_up_process</span>(k);</span><br><span class="line"> <span class="built_in">wait_for_completion</span>(&kthread->exited);</span><br><span class="line"> }</span><br><span class="line"> ret = k->exit_code;</span><br><span class="line"> <span class="built_in">put_task_struct</span>(k);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">trace_sched_kthread_stop_ret</span>(ret);</span><br><span class="line"> <span class="keyword">return</span> ret;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="2-进程-park"><a href="#2-进程-park" class="headerlink" title="2. 进程 park"></a>2. 进程 park</h2><p><code>smpboot_register_percpu_thread()</code> 用来创建 per_cpu 内核进程，所谓的 per_cpu 进程是指需要在每个 online cpu 上创建线程。比如执行 <code>stop_machine()</code> 中 cpu 同步操作的 migration 进程：</p>
<figure class="highlight apache"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="attribute">shell</span>@:/ $ ps | grep migration</span><br><span class="line"><span class="attribute">root</span> <span class="number">10</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> smpboot_th <span class="number">0000000000</span> S migration/<span class="number">0</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">11</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> smpboot_th <span class="number">0000000000</span> S migration/<span class="number">1</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">15</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">2</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">19</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">3</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">207</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">8</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">247</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">4</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">251</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">5</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">265</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">6</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">356</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">7</span></span><br><span class="line"><span class="attribute">root</span> <span class="number">2165</span> <span class="number">2</span> <span class="number">0</span> <span class="number">0</span> __kthread_ <span class="number">0000000000</span> R migration/<span class="number">9</span></span><br></pre></td></tr></table></figure>
<p>问题来了，既然 per_cpu 进程是和 cpu 绑定的，那么在 cpu hotplug 的时候，进程需要相应的 disable 和 enable。实现的方法可以有多种：</p>
<ul>
<li>动态的销毁和创建线程。缺点是开销比较大。</li>
<li>设置进程的 cpu 亲和力 <code>set_cpus_allowed_ptr()</code>。缺点是进程绑定的 cpu 如果被 down 掉，进程会迁移到其他 cpu 继续执行。</li>
</ul>
<p>为了克服上述方案的缺点，适配 per_cpu 进程的 cpu hotplug 操作，设计了 <code>kthread_park()</code>/<code>kthread_unpark()</code> 机制。</p>
<h3 id="2-1-smpboot-register-percpu-thread"><a href="#2-1-smpboot-register-percpu-thread" class="headerlink" title="2.1 smpboot_register_percpu_thread()"></a>2.1 <code>smpboot_register_percpu_thread()</code></h3><p>per_cpu 进程从代码上看，实际也是调用 <code>kthread_create()</code> 来创建的。</p>
<ul>
<li>kernel/smpboot.c:</li>
<li>kernel/kthread.c:</li>
</ul>
<p><img src="stop_smpboot_register.png" alt="smpboot_register_percpu_thread"></p>
<p>我们可以看到 smpboot_register 又增加了一层封装：<code>kthread()</code> -> <code>smpboot_thread_fn()</code> -> <code>ht->thread_fn()</code>，这种封装的使用可以参考 cpu_stop_threads。</p>
<ul>
<li>kernel/stop_machine.c:</li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">static</span> <span class="keyword">struct</span> <span class="title class_">smp_hotplug_thread</span> cpu_stop_threads = {</span><br><span class="line"> .store = &cpu_stopper_task,</span><br><span class="line"> .thread_should_run = cpu_stop_should_run,</span><br><span class="line"> .thread_fn = cpu_stopper_thread,</span><br><span class="line"> .thread_comm = <span class="string">"migration/%u"</span>,</span><br><span class="line"> .create = cpu_stop_create,</span><br><span class="line"> .setup = cpu_stop_unpark,</span><br><span class="line"> .park = cpu_stop_park,</span><br><span class="line"> .pre_unpark = cpu_stop_unpark,</span><br><span class="line"> .selfparking = <span class="literal">true</span>,</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">static</span> <span class="type">int</span> __init <span class="title">cpu_stop_init</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="type">unsigned</span> <span class="type">int</span> cpu;</span><br><span class="line"></span><br><span class="line"> for_each_possible_cpu(cpu) {</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">cpu_stopper</span> *stopper = &<span class="built_in">per_cpu</span>(cpu_stopper, cpu);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">spin_lock_init</span>(&stopper->lock);</span><br><span class="line"> <span class="built_in">INIT_LIST_HEAD</span>(&stopper->works);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="built_in">BUG_ON</span>(<span class="built_in">smpboot_register_percpu_thread</span>(&cpu_stop_threads));</span><br><span class="line"> stop_machine_initialized = <span class="literal">true</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>我们可以看到 <code>smpboot_thread_fn()</code> 循环中实现了对 park 的支持，具体实现 <code>kthread_should_park()</code>、<code>kthread_parkme()</code>、<code>kthread_park()</code>、<code>kthread_unpark()</code> 的代码分析：</p>
<ul>
<li>kernel/kthread.c:</li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">bool</span> <span class="title">kthread_should_park</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="comment">// (1) 判断进程所在 kthread 结构中的 KTHREAD_SHOULD_PARK 是否被置位</span></span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">test_bit</span>(KTHREAD_SHOULD_PARK, &<span class="built_in">to_kthread</span>(current)->flags);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">kthread_parkme</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> __kthread_parkme(<span class="built_in">to_kthread</span>(current));</span><br><span class="line">}</span><br><span class="line">| →</span><br><span class="line"><span class="type">static</span> <span class="type">void</span> __kthread_parkme(<span class="keyword">struct</span> kthread *self)</span><br><span class="line">{</span><br><span class="line"> <span class="comment">// (2) 如果当前进程的 KTHREAD_SHOULD_PARK 标志被置位 ,</span></span><br><span class="line"> <span class="comment">// 将当前进程进入 TASK_PARKED 的阻塞状态。</span></span><br><span class="line"> <span class="comment">// 如果 KTHREAD_SHOULD_PARK 不清除，</span></span><br><span class="line"> <span class="comment">// 就算被 wake_up 唤醒还是会循环进入 TASK_PARKED 的阻塞状态。</span></span><br><span class="line"> __set_current_state(TASK_PARKED);</span><br><span class="line"> <span class="keyword">while</span> (<span class="built_in">test_bit</span>(KTHREAD_SHOULD_PARK, &self->flags)) {</span><br><span class="line"> <span class="keyword">if</span> (!<span class="built_in">test_and_set_bit</span>(KTHREAD_IS_PARKED, &self->flags))</span><br><span class="line"> <span class="built_in">complete</span>(&self->parked);</span><br><span class="line"> <span class="built_in">schedule</span>();</span><br><span class="line"> __set_current_state(TASK_PARKED);</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">clear_bit</span>(KTHREAD_IS_PARKED, &self->flags);</span><br><span class="line"> __set_current_state(TASK_RUNNING);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">kthread_park</span><span class="params">(<span class="keyword">struct</span> task_struct *k)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">kthread</span> *kthread = <span class="built_in">to_live_kthread</span>(k);</span><br><span class="line"> <span class="type">int</span> ret = -ENOSYS;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (kthread) {</span><br><span class="line"> <span class="comment">// (3) 设置 KTHREAD_IS_PARKED 标志位，并且唤醒进程进入 park 状态</span></span><br><span class="line"> <span class="keyword">if</span> (!<span class="built_in">test_bit</span>(KTHREAD_IS_PARKED, &kthread->flags)) {</span><br><span class="line"> <span class="built_in">set_bit</span>(KTHREAD_SHOULD_PARK, &kthread->flags);</span><br><span class="line"> <span class="keyword">if</span> (k != current) {</span><br><span class="line"> <span class="built_in">wake_up_process</span>(k);</span><br><span class="line"> <span class="built_in">wait_for_completion</span>(&kthread->parked);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> ret = <span class="number">0</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> ret;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">kthread_unpark</span><span class="params">(<span class="keyword">struct</span> task_struct *k)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">kthread</span> *kthread = <span class="built_in">to_live_kthread</span>(k);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (kthread)</span><br><span class="line"> __kthread_unpark(k, kthread);</span><br><span class="line">}</span><br><span class="line">| →</span><br><span class="line"><span class="type">static</span> <span class="type">void</span> __kthread_unpark(<span class="keyword">struct</span> task_struct *k, <span class="keyword">struct</span> kthread *kthread)</span><br><span class="line">{</span><br><span class="line"> <span class="comment">// (4) 清除 KTHREAD_IS_PARKED 标志位</span></span><br><span class="line"> <span class="built_in">clear_bit</span>(KTHREAD_SHOULD_PARK, &kthread->flags);</span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * We clear the IS_PARKED bit here as we don't wait</span></span><br><span class="line"><span class="comment"> * until the task has left the park code. So if we'd</span></span><br><span class="line"><span class="comment"> * park before that happens we'd see the IS_PARKED bit</span></span><br><span class="line"><span class="comment"> * which might be about to be cleared.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="comment">// 如果进程已经被 park，并且 wake_up 唤醒进程</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">test_and_clear_bit</span>(KTHREAD_IS_PARKED, &kthread->flags)) {</span><br><span class="line"> <span class="comment">// 如果是 per_cpu 进程，重新绑定进程 cpu</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">test_bit</span>(KTHREAD_IS_PER_CPU, &kthread->flags))</span><br><span class="line"> __kthread_bind(k, kthread->cpu, TASK_PARKED);</span><br><span class="line"> <span class="built_in">wake_up_state</span>(k, TASK_PARKED);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3 id="2-2-cpu-hotplug-支持"><a href="#2-2-cpu-hotplug-支持" class="headerlink" title="2.2 cpu hotplug 支持"></a>2.2 cpu hotplug 支持</h3><p>我们前面说到 park 机制的主要目的是为了 per_cpu 进程支持 cpu hotplug，具体怎么响应热插拔事件呢？</p>
<ul>
<li>kernel/smpboot.c:</li>
</ul>
<p><img src="stop_park_hotplug.png" alt="park_hotplug"></p>
<h2 id="3-进程-freeze"><a href="#3-进程-freeze" class="headerlink" title="3. 进程 freeze"></a>3. 进程 freeze</h2><p>在系统进入 suspend 的时候，会尝试冻住一些进程，以避免一些进程无关操作影响系统的 suspend 状态。主要的流程如下：</p>
<ul>
<li>kernel/power/suspend.c:</li>
</ul>
<p><img src="stop_pm_suspend.png" alt="suspend_freeze_processes"></p>
<p>这 suspend_freeze 里面判断当前在那个阶段，有 3 个重要的变量：</p>
<ul>
<li>system_freezing_cnt - >0 表示系统全局的 freeze 开始；</li>
<li>pm_freezing - =true 表示用户进程 freeze 开始；</li>
<li>pm_nosig_freezing - =true 表示内核进程 freeze 开始；</li>
</ul>
<p>具体代码分析如下：</p>
<ul>
<li>kernel/power/process.c:</li>
<li>kernel/freezer.c:</li>
<li><code>suspend_freeze_processes()</code> -> <code>freeze_processes()</code> -> <code>try_to_freeze_tasks()</code> -> <code>freeze_task()</code></li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span 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class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br><span class="line">141</span><br><span class="line">142</span><br><span class="line">143</span><br><span class="line">144</span><br><span class="line">145</span><br><span class="line">146</span><br><span class="line">147</span><br><span class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br><span class="line">160</span><br><span class="line">161</span><br><span class="line">162</span><br><span class="line">163</span><br><span class="line">164</span><br><span class="line">165</span><br><span class="line">166</span><br><span class="line">167</span><br><span class="line">168</span><br><span class="line">169</span><br><span class="line">170</span><br><span class="line">171</span><br><span class="line">172</span><br><span class="line">173</span><br><span class="line">174</span><br><span class="line">175</span><br><span class="line">176</span><br><span class="line">177</span><br><span class="line">178</span><br><span class="line">179</span><br><span class="line">180</span><br><span class="line">181</span><br><span class="line">182</span><br><span class="line">183</span><br><span class="line">184</span><br><span class="line">185</span><br><span class="line">186</span><br><span class="line">187</span><br><span class="line">188</span><br><span class="line">189</span><br><span class="line">190</span><br><span class="line">191</span><br><span class="line">192</span><br><span class="line">193</span><br><span class="line">194</span><br><span class="line">195</span><br><span class="line">196</span><br><span class="line">197</span><br><span class="line">198</span><br><span class="line">199</span><br><span class="line">200</span><br><span class="line">201</span><br><span class="line">202</span><br><span class="line">203</span><br><span class="line">204</span><br><span class="line">205</span><br><span class="line">206</span><br><span class="line">207</span><br><span class="line">208</span><br><span class="line">209</span><br><span class="line">210</span><br><span class="line">211</span><br><span class="line">212</span><br><span class="line">213</span><br><span class="line">214</span><br><span class="line">215</span><br><span class="line">216</span><br><span class="line">217</span><br><span class="line">218</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">freeze_processes</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="type">int</span> error;</span><br><span class="line"> <span class="type">int</span> oom_kills_saved;</span><br><span class="line"></span><br><span class="line"> error = __usermodehelper_disable(UMH_FREEZING);</span><br><span class="line"> <span class="keyword">if</span> (error)</span><br><span class="line"> <span class="keyword">return</span> error;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (1) 置位 PF_SUSPEND_TASK，确保当前进程不会被 freeze</span></span><br><span class="line"> <span class="comment">/* Make sure this task doesn't get frozen */</span></span><br><span class="line"> current->flags |= PF_SUSPEND_TASK;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (2) 使用全局 freeze 标志 system_freezing_cnt</span></span><br><span class="line"> <span class="keyword">if</span> (!pm_freezing)</span><br><span class="line"> <span class="built_in">atomic_inc</span>(&system_freezing_cnt);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">pm_wakeup_clear</span>();</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"Freezing user space processes ... "</span>);</span><br><span class="line"> <span class="comment">// (3) 使用用户进程 freeze 标志 pm_freezing</span></span><br><span class="line"> pm_freezing = <span class="literal">true</span>;</span><br><span class="line"> oom_kills_saved = <span class="built_in">oom_kills_count</span>();</span><br><span class="line"> <span class="comment">// (4) freeze user_only 进程</span></span><br><span class="line"> <span class="comment">// 判断进程是否可以被 freeze，唤醒进程 freeze 自己</span></span><br><span class="line"> error = <span class="built_in">try_to_freeze_tasks</span>(<span class="literal">true</span>);</span><br><span class="line"> <span class="keyword">if</span> (!error) {</span><br><span class="line"> __usermodehelper_set_disable_depth(UMH_DISABLED);</span><br><span class="line"> <span class="built_in">oom_killer_disable</span>();</span><br><span class="line"></span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * There might have been an OOM kill while we were</span></span><br><span class="line"><span class="comment"> * freezing tasks and the killed task might be still</span></span><br><span class="line"><span class="comment"> * on the way out so we have to double check for race.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">oom_kills_count</span>() != oom_kills_saved &&</span><br><span class="line"> !<span class="built_in">check_frozen_processes</span>()) {</span><br><span class="line"> __usermodehelper_set_disable_depth(UMH_ENABLED);</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"OOM in progress."</span>);</span><br><span class="line"> error = -EBUSY;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"done."</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"\n"</span>);</span><br><span class="line"> <span class="built_in">BUG_ON</span>(<span class="built_in">in_atomic</span>());</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (error)</span><br><span class="line"> <span class="built_in">thaw_processes</span>();</span><br><span class="line"> <span class="keyword">return</span> error;</span><br><span class="line">}</span><br><span class="line">| →</span><br><span class="line"><span class="function"><span class="type">static</span> <span class="type">int</span> <span class="title">try_to_freeze_tasks</span><span class="params">(<span class="type">bool</span> user_only)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">task_struct</span> *g, *p;</span><br><span class="line"> <span class="type">unsigned</span> <span class="type">long</span> end_time;</span><br><span class="line"> <span class="type">unsigned</span> <span class="type">int</span> todo;</span><br><span class="line"> <span class="type">bool</span> wq_busy = <span class="literal">false</span>;</span><br><span class="line"> <span class="keyword">struct</span> <span class="title class_">timeval</span> start, end;</span><br><span class="line"> u64 elapsed_msecs64;</span><br><span class="line"> <span class="type">unsigned</span> <span class="type">int</span> elapsed_msecs;</span><br><span class="line"> <span class="type">bool</span> wakeup = <span class="literal">false</span>;</span><br><span class="line"> <span class="type">int</span> sleep_usecs = USEC_PER_MSEC;</span><br><span class="line"><span class="meta">#<span class="keyword">ifdef</span> CONFIG_PM_SLEEP</span></span><br><span class="line"> <span class="type">char</span> suspend_abort[MAX_SUSPEND_ABORT_LEN];</span><br><span class="line"><span class="meta">#<span class="keyword">endif</span></span></span><br><span class="line"></span><br><span class="line"> <span class="built_in">do_gettimeofday</span>(&start);</span><br><span class="line"></span><br><span class="line"> end_time = jiffies + <span class="built_in">msecs_to_jiffies</span>(freeze_timeout_msecs);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (4.1) 如果是 kernel freeze，</span></span><br><span class="line"> <span class="comment">// 停工有 WQ_FREEZABLE 标志的 workqueue</span></span><br><span class="line"> <span class="comment">// 将 wq 的 pwq->max_active 设置成 0，新的 work 不能被执行</span></span><br><span class="line"> <span class="keyword">if</span> (!user_only)</span><br><span class="line"> <span class="built_in">freeze_workqueues_begin</span>();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">while</span> (<span class="literal">true</span>) {</span><br><span class="line"> todo = <span class="number">0</span>;</span><br><span class="line"> <span class="built_in">read_lock</span>(&tasklist_lock);</span><br><span class="line"> <span class="comment">// (4.2) 对每个进程执行 freeze_task()</span></span><br><span class="line"> for_each_process_thread(g, p) {</span><br><span class="line"> <span class="keyword">if</span> (p == current || !<span class="built_in">freeze_task</span>(p))</span><br><span class="line"> <span class="keyword">continue</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (!<span class="built_in">freezer_should_skip</span>(p))</span><br><span class="line"> todo++;</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">read_unlock</span>(&tasklist_lock);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (4.3) 如果是 kernel freeze，</span></span><br><span class="line"> <span class="comment">// 判断停工的 workqueue 中残留的 work 有没有执行完</span></span><br><span class="line"> <span class="keyword">if</span> (!user_only) {</span><br><span class="line"> wq_busy = <span class="built_in">freeze_workqueues_busy</span>();</span><br><span class="line"> todo += wq_busy;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (!todo || <span class="built_in">time_after</span>(jiffies, end_time))</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">pm_wakeup_pending</span>()) {</span><br><span class="line"><span class="meta">#<span class="keyword">ifdef</span> CONFIG_PM_SLEEP</span></span><br><span class="line"> <span class="built_in">pm_get_active_wakeup_sources</span>(suspend_abort,</span><br><span class="line"> MAX_SUSPEND_ABORT_LEN);</span><br><span class="line"> <span class="built_in">log_suspend_abort_reason</span>(suspend_abort);</span><br><span class="line"><span class="meta">#<span class="keyword">endif</span></span></span><br><span class="line"> wakeup = <span class="literal">true</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * We need to retry, but first give the freezing tasks some</span></span><br><span class="line"><span class="comment"> * time to enter the refrigerator. Start with an initial</span></span><br><span class="line"><span class="comment"> * 1 ms sleep followed by exponential backoff until 8 ms.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="built_in">usleep_range</span>(sleep_usecs / <span class="number">2</span>, sleep_usecs);</span><br><span class="line"> <span class="keyword">if</span> (sleep_usecs < <span class="number">8</span> * USEC_PER_MSEC)</span><br><span class="line"> sleep_usecs *= <span class="number">2</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="built_in">do_gettimeofday</span>(&end);</span><br><span class="line"> elapsed_msecs64 = <span class="built_in">timeval_to_ns</span>(&end) - <span class="built_in">timeval_to_ns</span>(&start);</span><br><span class="line"> <span class="built_in">do_div</span>(elapsed_msecs64, NSEC_PER_MSEC);</span><br><span class="line"> elapsed_msecs = elapsed_msecs64;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (wakeup) {</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"\n"</span>);</span><br><span class="line"> <span class="built_in">printk</span>(KERN_ERR <span class="string">"Freezing of tasks aborted after %d.%03d seconds"</span>,</span><br><span class="line"> elapsed_msecs / <span class="number">1000</span>, elapsed_msecs % <span class="number">1000</span>);</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (todo) {</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"\n"</span>);</span><br><span class="line"> <span class="built_in">printk</span>(KERN_ERR <span class="string">"Freezing of tasks failed after %d.%03d seconds"</span></span><br><span class="line"> <span class="string">" (%d tasks refusing to freeze, wq_busy=%d):\n"</span>,</span><br><span class="line"> elapsed_msecs / <span class="number">1000</span>, elapsed_msecs % <span class="number">1000</span>,</span><br><span class="line"> todo - wq_busy, wq_busy);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">read_lock</span>(&tasklist_lock);</span><br><span class="line"> for_each_process_thread(g, p) {</span><br><span class="line"> <span class="keyword">if</span> (p != current && !<span class="built_in">freezer_should_skip</span>(p)</span><br><span class="line"> && <span class="built_in">freezing</span>(p) && !<span class="built_in">frozen</span>(p))</span><br><span class="line"> <span class="built_in">sched_show_task</span>(p);</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">read_unlock</span>(&tasklist_lock);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="built_in">printk</span>(<span class="string">"(elapsed %d.%03d seconds) "</span>, elapsed_msecs / <span class="number">1000</span>,</span><br><span class="line"> elapsed_msecs % <span class="number">1000</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> todo ? -EBUSY : <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line">|| →</span><br><span class="line"><span class="function"><span class="type">bool</span> <span class="title">freeze_task</span><span class="params">(<span class="keyword">struct</span> task_struct *p)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="type">unsigned</span> <span class="type">long</span> flags;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * This check can race with freezer_do_not_count, but worst case that</span></span><br><span class="line"><span class="comment"> * will result in an extra wakeup being sent to the task. It does not</span></span><br><span class="line"><span class="comment"> * race with freezer_count(), the barriers in freezer_count() and</span></span><br><span class="line"><span class="comment"> * freezer_should_skip() ensure that either freezer_count() sees</span></span><br><span class="line"><span class="comment"> * freezing == true in try_to_freeze() and freezes, or</span></span><br><span class="line"><span class="comment"> * freezer_should_skip() sees !PF_FREEZE_SKIP and freezes the task</span></span><br><span class="line"><span class="comment"> * normally.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">freezer_should_skip</span>(p))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">spin_lock_irqsave</span>(&freezer_lock, flags);</span><br><span class="line"> <span class="comment">// (4.2.1) 检查当前进程是否可以被 freeze，</span></span><br><span class="line"> <span class="comment">// 或者是否已经被 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (!<span class="built_in">freezing</span>(p) || <span class="built_in">frozen</span>(p)) {</span><br><span class="line"> <span class="built_in">spin_unlock_irqrestore</span>(&freezer_lock, flags);</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (4.2.2) 如果是用户进程，伪造一个 signal 发送给进程</span></span><br><span class="line"> <span class="keyword">if</span> (!(p->flags & PF_KTHREAD))</span><br><span class="line"> <span class="built_in">fake_signal_wake_up</span>(p);</span><br><span class="line"> <span class="comment">// (4.2.3) 如果是内核进程，wake_up 内核进程</span></span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> <span class="built_in">wake_up_state</span>(p, TASK_INTERRUPTIBLE);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">spin_unlock_irqrestore</span>(&freezer_lock, flags);</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">}</span><br><span class="line">||| →</span><br><span class="line"><span class="function"><span class="type">static</span> <span class="keyword">inline</span> <span class="type">bool</span> <span class="title">freezing</span><span class="params">(<span class="keyword">struct</span> task_struct *p)</span></span></span><br><span class="line"><span class="function"></span>{ 具体代码分析如下：</span><br><span class="line"></span><br><span class="line">- kernel/power/process.c:</span><br><span class="line">- kernel/freezer.c:</span><br><span class="line"> <span class="comment">// 如果 system_freezing_cnt 为 0，说明全局 freeze 还没有开始</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">likely</span>(!<span class="built_in">atomic_read</span>(&system_freezing_cnt)))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">freezing_slow_path</span>(p);</span><br><span class="line">}</span><br><span class="line">|||| →</span><br><span class="line"><span class="function"><span class="type">bool</span> <span class="title">freezing_slow_path</span><span class="params">(<span class="keyword">struct</span> task_struct *p)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="comment">// (PF_NOFREEZE | PF_SUSPEND_TASK) 当前进程不能被 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (p->flags & (PF_NOFREEZE | PF_SUSPEND_TASK))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">test_thread_flag</span>(TIF_MEMDIE))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 如果 pm_nosig_freezing 为 true，内核进程 freeze 已经开始，</span></span><br><span class="line"> <span class="comment">// 当前进程可以被 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (pm_nosig_freezing || <span class="built_in">cgroup_freezing</span>(p))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 如果 pm_freezing 为 true，且当前进程为用户进程</span></span><br><span class="line"> <span class="comment">// 当前进程可以被 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (pm_freezing && !(p->flags & PF_KTHREAD))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="3-1-用户进程-freeze"><a href="#3-1-用户进程-freeze" class="headerlink" title="3.1 用户进程 freeze"></a>3.1 用户进程 freeze</h3><p>freeze 用户态的进程利用了 signal 机制，系统 suspend 使能了 suspend 以后，调用 <code>fake_signal_wake_up()</code> 伪造一个信号唤醒进程，进程在 <code>ret_to_user()</code> -> <code>do_notify_resume()</code> -> <code>do_signal()</code> -> <code>get_signal()</code> -> <code>try_to_freeze()</code> 中 freeze 自己。</p>
<p>具体代码分析如下：</p>
<ul>
<li>kernel/freezer.c:</li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">static</span> <span class="keyword">inline</span> <span class="type">bool</span> <span class="title">try_to_freeze</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">if</span> (!(current->flags & PF_NOFREEZE))</span><br><span class="line"> <span class="built_in">debug_check_no_locks_held</span>();</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">try_to_freeze_unsafe</span>();</span><br><span class="line">}</span><br><span class="line">| →</span><br><span class="line"><span class="function"><span class="type">static</span> <span class="keyword">inline</span> <span class="type">bool</span> <span class="title">try_to_freeze_unsafe</span><span class="params">(<span class="type">void</span>)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="built_in">might_sleep</span>();</span><br><span class="line"> <span class="comment">// 当前进程是否可以被 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">likely</span>(!<span class="built_in">freezing</span>(current)))</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> <span class="comment">// 调用 __refrigerator() freeze 当前进程</span></span><br><span class="line"> <span class="keyword">return</span> __refrigerator(<span class="literal">false</span>);</span><br><span class="line">}</span><br><span class="line">|| →</span><br><span class="line"><span class="type">bool</span> __refrigerator(<span class="type">bool</span> check_kthr_stop)</span><br><span class="line">{</span><br><span class="line"> <span class="comment">/* Hmm, should we be allowed to suspend when there are realtime</span></span><br><span class="line"><span class="comment"> processes around? */</span></span><br><span class="line"> <span class="type">bool</span> was_frozen = <span class="literal">false</span>;</span><br><span class="line"> <span class="type">long</span> save = current->state;</span><br><span class="line"></span><br><span class="line"> <span class="built_in">pr_debug</span>(<span class="string">"%s entered refrigerator\n"</span>, current->comm);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> (;;) {</span><br><span class="line"> <span class="comment">// (1) 设置当前进程进入 TASK_UNINTERRUPTIBLE 阻塞状态</span></span><br><span class="line"> <span class="built_in">set_current_state</span>(TASK_UNINTERRUPTIBLE);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">spin_lock_irq</span>(&freezer_lock);</span><br><span class="line"> <span class="comment">// (2) 设置已经 freeze 标志 PF_FROZEN</span></span><br><span class="line"> current->flags |= PF_FROZEN;</span><br><span class="line"> <span class="comment">// (3) 如果当前进程已经不是 freeze 状态，</span></span><br><span class="line"> <span class="comment">// 退出 freeze</span></span><br><span class="line"> <span class="keyword">if</span> (!<span class="built_in">freezing</span>(current) ||</span><br><span class="line"> (check_kthr_stop && <span class="built_in">kthread_should_stop</span>()))</span><br><span class="line"> current->flags &= ~PF_FROZEN;</span><br><span class="line"> <span class="built_in">spin_unlock_irq</span>(&freezer_lock);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (!(current->flags & PF_FROZEN))</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> was_frozen = <span class="literal">true</span>;</span><br><span class="line"> <span class="built_in">schedule</span>();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="built_in">pr_debug</span>(<span class="string">"%s left refrigerator\n"</span>, current->comm);</span><br><span class="line"></span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * Restore saved task state before returning. The mb'd version</span></span><br><span class="line"><span class="comment"> * needs to be used; otherwise, it might silently break</span></span><br><span class="line"><span class="comment"> * synchronization which depends on ordered task state change.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="built_in">set_current_state</span>(save);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> was_frozen;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="3-2-内核进程-freeze"><a href="#3-2-内核进程-freeze" class="headerlink" title="3.2 内核进程 freeze"></a>3.2 内核进程 freeze</h3><p>内核进程对 freeze 的响应，有两个问题：</p>
<ul>
<li>wake_up_state(p, TASK_INTERRUPTIBLE) 能唤醒哪些内核进程。</li>
<li>内核进程怎么样来响应 freeze 状态，怎么样来 freeze 自己。</li>
</ul>
<p>如果进程阻塞在信号量、mutex 等内核同步机制上，wake_up_state 并不能解除阻塞。因为这些机制都有 while(1) 循环来判断条件，是否成立，不成立只是简单的唤醒随即又会进入阻塞睡眠状态。</p>
<ul>
<li>kernel/locking/mutex.c:</li>
<li><code>mutex_lock()</code> -> <code>__mutex_lock_common()</code></li>
</ul>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br></pre></td><td class="code"><pre><span class="line">__mutex_lock_common(<span class="keyword">struct</span> mutex *lock, <span class="type">long</span> state, <span class="type">unsigned</span> <span class="type">int</span> subclass,</span><br><span class="line"> <span class="keyword">struct</span> lockdep_map *nest_lock, <span class="type">unsigned</span> <span class="type">long</span> ip,</span><br><span class="line"> <span class="keyword">struct</span> ww_acquire_ctx *ww_ctx, <span class="type">const</span> <span class="type">bool</span> use_ww_ctx)</span><br><span class="line">{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> (;;) {</span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * Lets try to take the lock again - this is needed even if</span></span><br><span class="line"><span class="comment"> * we get here for the first time (shortly after failing to</span></span><br><span class="line"><span class="comment"> * acquire the lock), to make sure that we get a wakeup once</span></span><br><span class="line"><span class="comment"> * it's unlocked. Later on, if we sleep, this is the</span></span><br><span class="line"><span class="comment"> * operation that gives us the lock. We xchg it to -1, so</span></span><br><span class="line"><span class="comment"> * that when we release the lock, we properly wake up the</span></span><br><span class="line"><span class="comment"> * other waiters. We only attempt the xchg if the count is</span></span><br><span class="line"><span class="comment"> * non-negative in order to avoid unnecessary xchg operations:</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="comment">// (1) 如果 mutex_lock 条件成立，才退出</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">atomic_read</span>(&lock->count) >= <span class="number">0</span> &&</span><br><span class="line"> (<span class="built_in">atomic_xchg</span>(&lock->count, <span class="number">-1</span>) == <span class="number">1</span>))</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (2) 如果如果有信号阻塞，也退出</span></span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> * got a signal? (This code gets eliminated in the</span></span><br><span class="line"><span class="comment"> * TASK_UNINTERRUPTIBLE case.)</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">unlikely</span>(<span class="built_in">signal_pending_state</span>(state, task))) {</span><br><span class="line"> ret = -EINTR;</span><br><span class="line"> <span class="keyword">goto</span> err;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (use_ww_ctx && ww_ctx->acquired > <span class="number">0</span>) {</span><br><span class="line"> ret = __mutex_lock_check_stamp(lock, ww_ctx);</span><br><span class="line"> <span class="keyword">if</span> (ret)</span><br><span class="line"> <span class="keyword">goto</span> err;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// (3) 否则继续进入阻塞休眠状态</span></span><br><span class="line"> __set_task_state(task, state);</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* didn't get the lock, go to sleep: */</span></span><br><span class="line"> <span class="built_in">spin_unlock_mutex</span>(&lock->wait_lock, flags);</span><br><span class="line"> <span class="built_in">schedule_preempt_disabled</span>();</span><br><span class="line"> <span class="built_in">spin_lock_mutex</span>(&lock->wait_lock, flags);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>所以 <code>wake_up_state()</code> 只能唤醒这种简单阻塞的内核进程，而对于阻塞在内核同步机制上是无能无力的：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">user_thread</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">while</span>(<span class="number">1</span>)</span><br><span class="line"> {</span><br><span class="line"> <span class="built_in">set_current_state</span>(TASK_UNINTERRUPTIBLE);</span><br><span class="line"> <span class="built_in">schedule</span>();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>内核进程响应 freeze 操作，也必须显式的调用 <code>try_to_freeze()</code> 或者 <code>kthread_freezable_should_stop()</code> 来 freeze 自己：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">user_thread</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"> <span class="keyword">while</span> (!<span class="built_in">kthread_should_stop</span>()) {</span><br><span class="line"></span><br><span class="line"> <span class="built_in">try_to_freeze</span>();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>所以从代码逻辑上看内核进程 freeze，并不会 freeze 所有内核进程，只 freeze 了 2 部分：一部分是设置了 WQ_FREEZABLE 标志的 workqueue，另一部分是内核进程主动调用 <code>try_to_freeze()</code> 并且在架构上设计的可以响应 freeze。</p>
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