SystemTap入门指南：Linux调试神器

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SystemTap 是一款强大的 Linux 调试工具，专为深入监控和分析系统行为而设计。作为一款开源软件，它遵循 GNU General Public License（GPL）版本 2 的条款，允许用户自由地分发、修改和使用。SystemTap 3.2 版本的《SystemTap入门指南》由 Don Domingo 和 William Cohen 合著，他们分别来自 Red Hat，这本指南是为初学者量身打造的，旨在帮助读者掌握如何利用 SystemTap 来监控和调试 Linux 系统的各个组件。该指南的第 3.2 版本详细介绍了如何使用 SystemTap 进行以下操作： 1. **基础介绍**：首先，读者将学习到 SystemTap 的基本概念，包括其设计理念、安装方法以及与传统调试工具的区别，强调了它在实时性能分析、日志记录和性能瓶颈查找方面的优势。 2. **系统监控**：指南会指导用户设置脚本，通过 SystemTap 来监视系统活动，如进程管理、内存使用、网络通信、磁盘 I/O 等。这包括探针（probes）的编写和配置，以及如何使用 ` stap` 命令行工具进行实时监控。 3. **定制探针**：为了满足特定需求，读者将学习如何创建自定义探针，通过扩展 SystemTap API 来实现对特定内核模块或应用程序的深入监控。 4. **数据收集和分析**：指南涵盖了如何解析和处理 SystemTap 收集的数据，包括数据输出格式、过滤器的使用以及与外部工具（如 awk、grep 或 Python）的集成，以便进行深入分析。 5. **实战示例**：书中提供了一系列实用的案例，帮助读者通过实际操作掌握 SystemTap 的各种功能，包括系统启动过程、服务管理、系统调用跟踪等。 6. **版权和许可**：最后，版权信息和 GNU GPL 的细节被明确提及，确保用户了解其在使用 SystemTap 时的权益和义务。《SystemTap入门指南》版 3.2 是一本全面且易懂的教程，适合任何想要提升 Linux 系统理解和调试能力的开发者或运维人员，无论是为了日常问题排查还是长期性能优化，SystemTap 都是不可或缺的工具。

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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