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‎linux/kernel-sysrq.md‎

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---
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title: "Linux Magic System Request Key Hacks"
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date: "2024-12-05T09:44:34+07:00"
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tags: ["linux", "tips", "tech"]
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comments: true
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toc: true
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draft: false
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---
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Source: <https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html>
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## Giới thiệu
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Trong quá trình vận hành, bạn đã bao giờ gặp tình trạng hệ thống Linux của mình bị "treo" hoặc không phản hồi? Khi đó, hãy sử dụng **Magic System Request Key (SysRq)** để được cứu rỗi. Vậy nó là gì và có thể làm gì?
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SysRq là một tính năng của Linux, cho phép người dùng gửi "tín hiệu cầu cứu" trực tiếp đến kernel của hệ điều hành.
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{{< figure class="figure" caption="sysrq - from trufflesecurity.com" src="https://framerusercontent.com/images/wjLSwytVCtdnGhq2xK8m6qSo4.png" >}}
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## Cấu hình SysRq
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Để cấu hình SysRq, bạn có thể sử dụng command sau:
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```shell
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echo "number" >/proc/sys/kernel/sysrq
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```
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Giá trị của "number" có thể nằm trong các trường hợp sau:
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```shell
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0 - disable sysrq completely
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1 - enable all functions of sysrq
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>1 – bitmask to allow specific sysrq functions
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2 = 0x2 - enable control of console logging level
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4 = 0x4 - enable control of keyboard (SAK, unraw)
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8 = 0x8 - enable debugging dumps of processes etc.
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16 = 0x10 - enable sync command
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32 = 0x20 - enable remount read-only
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64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
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128 = 0x80 - allow reboot/poweroff
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256 = 0x100 - allow nicing of all RT tasks
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```
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Kiểm tra giá trị hiện tại của sysrq:
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```shell
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root@vm1:/home/kien# cat /proc/sys/kernel/sysrq
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176
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# 176 không match với giá trị nào, well, thực ra 176 ở đây là 16+32+128 = 176
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#
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# 16 = 0x10 - enable sync command
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# 32 = 0x20 - enable remount read-only
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# 128 = 0x80 - allow reboot/poweroff
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# Để test, bật hết lên cho đơn giản
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root@vm1:/home/kien# root@vm1:/home/kien# echo 1 > /proc/sys/kernel/sysrq
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```
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## Cách sử dụng
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Bạn có thể sử dụng tính năng bằng cách ấn tổ hợp phím (tùy thuộc hệ điều hành, đối với x86 là `ALT-SysRq-<command key>`, bàn phím của bạn thường sẽ có SysRq keyboard, để ý nhé) hoặc echo ký tự commands vào `/proc/sysrq-trigger`.
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```shell
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echo <command key> > /proc/sysrq-trigger
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```
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{{< details title="Danh sách command key (ấn để show all)" open=false >}}
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<table class="docutils align-default">
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<thead>
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<tr class="row-odd"><th class="head"><p>Command</p></th>
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<th class="head"><p>Function</p></th>
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</tr>
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</thead>
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<tbody>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">b</span></code></p></td>
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<td><p>Will immediately reboot the system without syncing or unmounting
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your disks.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">c</span></code></p></td>
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<td><p>Will perform a system crash and a crashdump will be taken
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if configured.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">d</span></code></p></td>
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<td><p>Shows all locks that are held.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">e</span></code></p></td>
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<td><p>Send a SIGTERM to all processes, except for init.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">f</span></code></p></td>
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<td><p>Will call the oom killer to kill a memory hog process, but do not
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panic if nothing can be killed.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">g</span></code></p></td>
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<td><p>Used by kgdb (kernel debugger)</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">h</span></code></p></td>
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<td><p>Will display help (actually any other key than those listed
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here will display help. but <code class="docutils literal notranslate"><span class="pre">h</span></code> is easy to remember :-)</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">i</span></code></p></td>
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<td><p>Send a SIGKILL to all processes, except for init.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">j</span></code></p></td>
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<td><p>Forcibly “Just thaw it” - filesystems frozen by the FIFREEZE ioctl.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">k</span></code></p></td>
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<td><p>Secure Access Key (SAK) Kills all programs on the current virtual
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console. NOTE: See important comments below in SAK section.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">l</span></code></p></td>
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<td><p>Shows a stack backtrace for all active CPUs.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">m</span></code></p></td>
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<td><p>Will dump current memory info to your console.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">n</span></code></p></td>
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<td><p>Used to make RT tasks nice-able</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">o</span></code></p></td>
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<td><p>Will shut your system off (if configured and supported).</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">p</span></code></p></td>
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<td><p>Will dump the current registers and flags to your console.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">q</span></code></p></td>
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<td><p>Will dump per CPU lists of all armed hrtimers (but NOT regular
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timer_list timers) and detailed information about all
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clockevent devices.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">r</span></code></p></td>
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<td><p>Turns off keyboard raw mode and sets it to XLATE.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">s</span></code></p></td>
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<td><p>Will attempt to sync all mounted filesystems.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">t</span></code></p></td>
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<td><p>Will dump a list of current tasks and their information to your
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console.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">u</span></code></p></td>
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<td><p>Will attempt to remount all mounted filesystems read-only.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">v</span></code></p></td>
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<td><p>Forcefully restores framebuffer console</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">v</span></code></p></td>
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<td><p>Causes ETM buffer dump [ARM-specific]</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">w</span></code></p></td>
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<td><p>Dumps tasks that are in uninterruptible (blocked) state.</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">x</span></code></p></td>
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<td><p>Used by xmon interface on ppc/powerpc platforms.
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Show global PMU Registers on sparc64.
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Dump all TLB entries on MIPS.</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">y</span></code></p></td>
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<td><p>Show global CPU Registers [SPARC-64 specific]</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">z</span></code></p></td>
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<td><p>Dump the ftrace buffer</p></td>
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</tr>
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<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">0</span></code>-<code class="docutils literal notranslate"><span class="pre">9</span></code></p></td>
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<td><p>Sets the console log level, controlling which kernel messages
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will be printed to your console. (<code class="docutils literal notranslate"><span class="pre">0</span></code>, for example would make
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it so that only emergency messages like PANICs or OOPSes would
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make it to your console.)</p></td>
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</tr>
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<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">R</span></code></p></td>
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<td><p>Replay the kernel log messages on consoles.</p></td>
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</tr>
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</tbody>
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</table>
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{{</ details >}}
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## Một số command keys hữu ích
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> Để dễ hình dung, ví dụ dưới đây sẽ sử dụng phương án echo ký tự vào /proc/sysrq-trigger
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### Poweroff
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```shell
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root@vm1:/home/kien# echo o > /proc/sysrq-trigger
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```
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### Reboot
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```shell
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root@vm1:/home/kien# echo b > /proc/sysrq-trigger
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```
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### Crash
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Trigger crashdump thủ công nếu hệ thống bị treo. Đây cũng là một cách hay để giả lập kernel crashdump.
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```shell
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root@vm1:/home/kien# echo c > /proc/sysrq-trigger
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```
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### Đồng bộ filesystems
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```shell
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root@vm1:/home/kien# echo s > /proc/sysrq-trigger
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```
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### Remount filesystem read-only
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```shell
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root@vm1:/home/kien# echo u > /proc/sysrq-trigger
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root@vm1:/home/kien# touch abc
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touch: cannot touch 'abc': Read-only file system
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```
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### Kill tất cả processes trừ tiến trình init
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Command key này đặc biệt hữu ích nếu bạn có tiến trình không thể kill, đặc biệt nếu tiến trình đó liên tục spawning ra các tiến trình khác.
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```shell
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# Linux gửi SIGTERM đến tất cả các processes, trừ init
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root@vm1:/home/kien# echo e > /proc/sysrq-trigger
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# Linux gửi SIGKILL đến tất cả các processes, trừ init
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root@vm1:/home/kien# echo i > /proc/sysrq-trigger
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```
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### Gọi OOM Killer
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OOM Killer được gọi và hoàn thành nhiệm vụ của nó - kill tiến trình gây high memory usage.
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```shell
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root@vm1:/home/kien# echo f > /proc/sysrq-trigger
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# Check kern.log để kiểm tra log
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3585:Dec 5 03:31:40 vm1 kernel: [ 195.899186] sysrq: Manual OOM execution
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3586:Dec 5 03:31:40 vm1 kernel: [ 195.901176] kworker/0:1 invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=-1, oom_score_adj=0
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```
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### Xem danh sách blocked state processes
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```shell
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root@vm1:/home/kien# echo w > /proc/sysrq-trigger
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# Check kern.log để kiểm tra
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Dec 5 03:33:02 vm1 kernel: [ 277.781446] sysrq: Show Blocked State
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```

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