Chapter 2. Using SystemTap

This chapter instructs users how to install SystemTap, and provides an introduction on how to run SystemTap scripts.

2.1. Installation and Setup

To deploy SystemTap, SystemTap packages along with the corresponding set of -devel, -debuginfo and -debuginfo-common-arch packages for the kernel need to be installed. To use SystemTap on more than one kernel where a system has multiple kernels installed, install the -devel and -debuginfo packages for each of those kernel versions.

These procedures will be discussed in detail in the following sections.

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 RPMs:

systemtap
systemtap-runtime

Assuming that yum is installed in the system, these two rpms can be installed with yum install systemtap systemtap-runtime. Install the required kernel information RPMs before using SystemTap.

2.1.2. Installing Required Kernel Information RPMs

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

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

Likewise, the necessary packages for the PAE kernel would be kernel-PAE-debuginfo, kernel-PAE-debuginfo-common-arch ,and kernel-PAE-devel.

To determine what kernel your system is currently using, use:

uname -r

For example, if you wish to use SystemTap on kernel version 2.6.32-53.el6 on an i686 machine, then you would need to download and install the following RPMs:

kernel-debuginfo-2.6.32-53.el6.i686.rpm
kernel-debuginfo-common-i686-2.6.32-53.el6.i686.rpm
kernel-devel-2.6.32-53.el6.i686.rpm

Important

The version, variant, and architecture of the -devel, -debuginfo and -debuginfo-common-arch packages must match the kernel to be probed with SystemTap exactly.

The easiest way to install the required kernel information packages is through yum install and debuginfo-install. Included with later versions of the yum-utils package is the debuginfo-install (for example, version 1.1.10). Also, debuginfo-install requires an appropriate yum repository from which to download and install -debuginfo/-debuginfo-common-arch packages.

Most required kernel packages can be found at ftp://ftp.redhat.com/pub/redhat/linux/enterprise/; navigate there until the the appropriate Debuginfo directory for the system is found.. Configure yum accordingly by adding a new "debug" yum repository file under /etc/yum.repos.d containing the following lines:

[rhel-debuginfo]
name=Red Hat Enterprise Linux $releasever - $basearch - Debug
baseurl=ftp://ftp.redhat.com/pub/redhat/linux/enterprise/$releasever/en/os/$basearch/Debuginfo/
enabled=1

After configuring yum with the appropriate repository, install the required -devel, -debuginfo, and -debuginfo-common-arch packages for the kernel by running the following commands:

yum install kernelname-devel-version
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-2.6.32-53.el6 kernel, run:

yum install kernel-PAE-devel-2.6.32-53.el6
debuginfo-install kernel-PAE-2.6.32-53.el6

If yum and yum-utils are not installed (and unable to be installed), manually download and install the required kernel information packages. To generate the URL from which to download the required packages, use the following script:

rheldebugurl.sh

#! /bin/bash
pkg="redhat-release-server"
releasever=`rpm -q --qf "%{version}" $pkg`
base=`uname -m`
echo "ftp://ftp.redhat.com/pub/redhat/linux/\
enterprise/$releasever/en/os/$base/Debuginfo"

Once the required packages to the machine have been manually downloaded, install the RPMs by running rpm --force -ivh package_names.

2.1.3. Initial Testing

If the kernel to be probed with SystemTap is currently being used, it is possible to immediately test whether the deployment was successful. If a different kernel is to be probed, reboot and load the appropriate kernel.

To start the test, run the command stap -v -e 'probe vfs.read {printf("read performed\n"); exit()}'. This command simply instructs SystemTap to print read performed then exit properly once a virtual file system read is detected. If the SystemTap deployment was successful, you should get 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 (i.e. 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).