SystemTap初学者指南：构建Linux内核调试环境

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"SystemTap 是一种强大的工具，用于在 Linux 系统中进行动态监控和调试。SystemTap Beginners Guide 是一份权威的官方文档，旨在引导初学者了解 SystemTap 的实现原理，并教授如何构建和使用 SystemTap 环境来调试 Linux 内核。该指南由 Don Domingo 和 William Cohen 编写，适用于 SystemTap 3.0 版本。 SystemTap 提供了一种高级语言，使得用户无需修改内核源代码或使用复杂的探针技术，就能编写脚本来收集系统运行时的信息。它允许你深入到内核层面，追踪系统行为，如进程活动、内存使用、系统调用等。这种能力对于理解系统性能问题、排查故障以及优化系统配置至关重要。在本指南中，读者将学习到： 1. **SystemTap 基础**：了解 SystemTap 的概念和架构，包括其如何与内核交互以及如何编译和执行 SystemTap 脚本。 2. **安装与配置**：指导如何在不同 Linux 发行版上安装 SystemTap 及其依赖，包括构建工具链和内核模块。 3. **编写 SystemTap 脚本**：学习 SystemTap 脚本语法，包括声明变量、定义探针、过滤条件和数据处理函数。 4. **调试技巧**：学习如何调试 SystemTap 脚本，找出错误并优化性能。 5. **应用示例**：通过实例展示如何使用 SystemTap 监控 CPU 使用率、磁盘 I/O、网络流量等系统指标。 6. **高级特性**：探讨更复杂的应用场景，如分析锁竞争、跟踪内存分配和释放，以及自定义事件。 7. **许可证信息**：SystemTap 遵循 GNU General Public License v2，文档也提供了相应的许可条款。 8. **获取帮助**：了解如何查找和参与社区，获取关于 SystemTap 的额外支持和资源。 SystemTap Beginners Guide 是一个全面的入门教程，无论你是系统管理员、开发者还是性能分析师，都能从中受益，提升你在 Linux 环境中的监控和诊断能力。" 这份文档详细地阐述了如何利用 SystemTap 来深入了解 Linux 系统的行为，是学习和掌握这一强大工具的理想起点。通过实践和探索，你可以更加熟练地运用 SystemTap 解决实际工作中遇到的问题，提高系统管理和维护的效率。

Chapter 2. Using SystemTap

Members of this group can use the stap command to run SystemTap scripts, or staprun to run

SystemTap instrumentation modules.

Running the stap command involves compiling SystemTap scripts into kernel modules and loading

them into the kernel. This operation requires elevated privileges to the system, which are granted

to stapdev members. Unfortunately, such privileges also grant effective root access to stapdev

members. As a consequence, only grant stapdev group membership to users whom you can trust

with root access.

stapusr

Members of this group can only use the staprun command to run SystemTap instrumentation

modules. In addition, they can only run modules from the /lib/modules/kernel_version/

systemtap/ directory. Note that this directory must be owned only by the root user, and must only

be writable by the root user.

The stap command reads a SystemTap script either from a file, or from standard input. To tell stap to

read a SystemTap script from a file, specify the file name on the command line:

stap file_name

To instruct stap to read a SystemTap script from standard input, use the - switch instead of the file

name. Note that any command-line options you wish to use must be inserted before the - switch. For

example, to make the output of the stap command more verbose, type:

echo "probe timer.s(1) {exit()}" | stap -v -

Below is a list of commonly used stap options:

-v

Makes the output of the SystemTap session more verbose. You can repeat this option multiple times

to provide more details on the script's execution, for example:

stap -vvv script.stp

This option is particularly useful if you encounter any errors in running the script. For more information

about common SystemTap script errors, refer to Chapter 6, Understanding SystemTap Errors.

-o file_name

Sends the standard output to a file named file_name.

-S size,count

Limits the maximum size of output files to size megabytes and the maximum number of stored files to

count. This option implements logrotate operations for SystemTap and the resulting file names have a

sequence number suffix.

-x process_id

Sets the SystemTap handler function target() to the specified process ID. For more information

about target(), refer to SystemTap Functions.

SystemTap Flight Recorder Mode

-c 'command'

Sets the SystemTap handler function target() to the specified command and runs the SystemTap

instrumentation for the duration of this command. For more information about target(), refer to

SystemTap Functions.

-e 'script'

Uses script rather than a file as input for the SystemTap translator.

-F

Uses SystemTap's flight recorder mode and makes the script a background process. For more

information about flight recorder mode, refer to Section 2.3.1, “SystemTap Flight Recorder Mode”.

For more information about the stap command, refer to the stap(1) man page. For more information

about the staprun command and cross-instrumentation, refer to Section 2.2, “Generating

Instrumentation for Other Computers” or the staprun(8) man page.

2.3.1. SystemTap Flight Recorder Mode

SystemTap's flight recorder mode allows you to run a SystemTap script for long periods of time and just

focus on recent output. The flight recorder mode limits the amount of output generated.

There are two variations of the flight recorder mode: in-memory and file mode. In both cases, the

SystemTap script runs as a background process.

2.3.1.1. In-memory Flight Recorder

When flight recorder mode is used without a file name, SystemTap uses a buffer in kernel memory to

store the output of the script. Once the SystemTap instrumentation module is loaded and the probes start

running, the instrumentation detaches and is put in the background. When the interesting event occurs,

you can reattach to the instrumentation to see the recent output in the memory buffer and any continuing

output.

To run a SystemTap script by using the flight recorder in-memory mode, run the stap command with the

-F command line option:

stap -F iotime.stp

Once the script starts, stap prints a message similar to the following to provide you with the command to

reconnect to the running script:

Disconnecting from systemtap module.

To reconnect, type "staprun -A stap_5dd0073edcb1f13f7565d8c343063e68_19556"

When the interesting event occurs, run the following command to connect to the currently running script,

output the recent data in the memory buffer, and get continuing output:

staprun -A stap_5dd0073edcb1f13f7565d8c343063e68_19556

Chapter 3.

Understanding How SystemTap Works

SystemTap allows users to write and reuse simple scripts to deeply examine the activities of a running

Linux system. These scripts can be designed to extract data, filter it, and summarize it quickly (and

safely), enabling the diagnosis of complex performance (or even functional) problems.

The essential idea behind a SystemTap script is to name events, and to give them handlers. When

SystemTap runs the script, SystemTap monitors for the event; once the event occurs, the Linux kernel

then runs the handler as a quick sub-routine, then resumes.

There are several kind of events; entering/exiting a function, timer expiration, session termination, etc.

A handler is a series of script language statements that specify the work to be done whenever the event

occurs. This work normally includes extracting data from the event context, storing them into internal

variables, and printing results.

3.1. Architecture

A SystemTap session begins when you run a SystemTap script. This session occurs in the following

fashion:

Procedure 3.1. SystemTap Session

1. First, SystemTap checks the script against the existing tapset library (normally in /usr/share/

systemtap/tapset/ for any tapsets used. SystemTap will then substitute any located tapsets with

their corresponding definitions in the tapset library.

2. SystemTap then translates the script to C, running the system C compiler to create a kernel

module from it. The tools that perform this step are contained in the systemtap package (refer to

Section 2.1.1, “Installing SystemTap” for more information).

3. SystemTap loads the module, then enables all the probes (events and handlers) in the script. The

staprun in the systemtap-runtime package (refer to Section 2.1.1, “Installing SystemTap” for

more information) provides this functionality.

4. As the events occur, their corresponding handlers are executed.

5. Once the SystemTap session is terminated, the probes are disabled, and the kernel module is

unloaded.

This sequence is driven from a single command-line program: stap. This program is SystemTap's main

front-end tool. For more information about stap, refer to man stap (once SystemTap is properly installed

on your machine).

3.2. SystemTap Scripts

For the most part, SystemTap scripts are the foundation of each SystemTap session. SystemTap

scripts instruct SystemTap on what type of information to collect, and what to do once that information is

collected.

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