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# ⁠Chapter 2. Using SystemTap

This chapter documents how to install SystemTap in the system and explains how to use the `stap` utility to run SystemTap scripts.

## ⁠2.1. Installation and Setup

To deploy SystemTap, install the SystemTap packages along with the corresponding set of -devel, -debuginfo, and -debuginfo-common packages for your kernel. If your system has multiple kernels installed and you intend to use SystemTap on more than one of them, also install the -devel and -debuginfo packages for each of those kernel versions.
The following sections discuss the installation procedures in greater detail.

Important

Many users confuse -debuginfo with -debug. Remember that the deployment of SystemTap requires the installation of the -debuginfo package of the kernel, not the -debug version of the kernel.

### ⁠2.1.1. Installing SystemTap

To deploy SystemTap, install the following RPM packages:

- systemtap
- systemtap-runtime

To do so, run the following command as root:

    yum install systemtap systemtap-runtime

Note that before using SystemTap, you still need to install the required kernel information packages. On modern systems, run the following command as root to install these packages:

    stap-prep

If this command does not work, try manual installation as described below.

### ⁠2.1.2. Installing Required Kernel Information Packages Manually

SystemTap needs information about the kernel in order to place instrumentation in it (in other words, probe it). This information also allows SystemTap to generate the code for the instrumentation.
The required information is contained in the matching -devel, -debuginfo, and -debuginfo-common packages for your kernel. The necessary -devel and -debuginfo packages for the ordinary "vanilla" kernel are as follows:

- kernel-debuginfo
- kernel-debuginfo-common
- kernel-devel

Likewise, the necessary packages for the PAE kernel are kernel-PAE-debuginfo, kernel-PAE-debuginfo-common, and kernel-PAE-devel.
To determine what kernel your system is currently using, use:

    uname -r

For example, if you intend to use SystemTap on kernel version 2.6.18-53.el5 on an i686 machine, download and install the following RPM packages:

- kernel-debuginfo-2.6.18-53.1.13.el5.i686.rpm
- kernel-debuginfo-common-2.6.18-53.1.13.el5.i686.rpm
- kernel-devel-2.6.18-53.1.13.el5.i686.rpm

Important

The version, variant, and architecture of the -devel, -debuginfo and -debuginfo-common packages must match the kernel you wish to probe with SystemTap exactly.
The easiest way to install the required kernel information packages is through `yum install` and `dnf debuginfo-install` commands. You can install the required -devel, -debuginfo, and -debuginfo-common packages for your kernel.
When the appropriate software repositories are enabled, install the corresponding packages for a specific kernel with the following commands:

    yum install kernelname-devel-version
    dnf debuginfo-install kernelname-version

Replace kernelname with the appropriate kernel variant name (for example, `kernel-PAE`), and version with the target kernel's version. For example, to install the required kernel information packages for the `kernel-PAE-2.6.18-53.1.13.el5` kernel, run:

    yum install kernel-PAE-devel-2.6.18-53.1.13.el5
    dnf debuginfo-install kernel-PAE-2.6.18-53.1.13.el5

Once you have manually downloaded the required packages to the machine, run the following command as root to install them:

    rpm --force -ivh package_names

### ⁠2.1.3. Initial Testing

If you are currently using the kernel you intend to probe with SystemTap, you can immediately test whether the deployment was successful. If not, you restart the system and load the appropriate kernel.
To start the test, run the following command:

    stap -v -e 'probe vfs.read {printf("read performed\n"); exit()}'

This command instructs SystemTap to print `read performed` and then exit properly once a virtual file system read is detected. If the SystemTap deployment was successful, it prints output similar to the following:

    Pass 1: parsed user script and 45 library script(s) in 340usr/0sys/358real ms.
    Pass 2: analyzed script: 1 probe(s), 1 function(s), 0 embed(s), 0 global(s) in 290usr/260sys/568real ms.
    Pass 3: translated to C into "/tmp/stapiArgLX/stap_e5886fa50499994e6a87aacdc43cd392_399.c" in 490usr/430sys/938real ms.
    Pass 4: compiled C into "stap_e5886fa50499994e6a87aacdc43cd392_399.ko" in 3310usr/430sys/3714real ms.
    Pass 5: starting run.
    read performed
    Pass 5: run completed in 10usr/40sys/73real ms.

The last three lines of the output (beginning with `Pass 5`) indicate that SystemTap was able to successfully create the instrumentation to probe the kernel, run the instrumentation, detect the event being probed (in this case, a virtual file system read), and execute a valid handler (print text then close it with no errors).

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