RHEL 6 SystemTap Guide - 2.2. Generating Instrumentation for Other Computers

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2.2. Generating Instrumentation for Other Computers

When users run a SystemTap script, a kernel module is built out of that script. SystemTap then loads the module into the kernel, allowing it to extract the specified data directly from the kernel (refer to Procedure 3.1, “SystemTap Session” in Section 3.1, “Architecture” for more information).

Normally, SystemTap scripts can only be run on systems where SystemTap is deployed (as in Section 2.1, “Installation and Setup”). This could mean that to run SystemTap on ten systems, SystemTap needs to be deployed on all those systems. In some cases, this may be neither feasible nor desired. For instance, corporate policy may prohibit an administrator from installing RPMs that provide compilers or debug information on specific machines, which will prevent the deployment of SystemTap.

To work around this, use cross-instrumentation. Cross-instrumentation is the process of generating SystemTap instrumentation modules from a SystemTap script on one computer to be used on another computer. This process offers the following benefits:

The kernel information packages for various machines can be installed on a single host machine.
Each target machine only needs one RPM to be installed to use the generated SystemTap instrumentation module: systemtap-runtime.

Note

For the sake of simplicity, the following terms will be used throughout this section:

instrumentation module — the kernel module built from a SystemTap script; i.e. the SystemTap module is built on the host system, and will be loaded on the target kernel of target system.
host system — the system on which the instrumentation modules (from SystemTap scripts) are compiled, to be loaded on target systems.
target system — the system in which the instrumentation module is being built (from SystemTap scripts).
target kernel — the kernel of the target system. This is the kernel which loads/runs the instrumentation module.

Procedure 2.1. Configuring a Host System and Target Systems

Install the systemtap-runtime RPM on each target system.
Determine the kernel running on each target system by running uname -r on each target system.
Install SystemTap on the host system. The instrumentation module will be built for the target systems on the host system. For instructions on how to install SystemTap, refer to Section 2.1.1, “Installing SystemTap”.
Using the target kernel version determined earlier, install the target kernel and related RPMs on the host system by the method described in Section 2.1.2, “Installing Required Kernel Information RPMs”. If multiple target systems use different target kernels, repeat this step for each different kernel used on the target systems.

After performing Procedure 2.1, “Configuring a Host System and Target Systems”, the instrumentation module (for any target system) can now be built on the host system.

To build the instrumentation module, run the following command on the host system (be sure to specify the appropriate values):

stap -r kernel_version script -m module_name -p4

Here, kernel_version refers to the version of the target kernel (the output of uname -r on the target machine), script refers to the script to be converted into an instrumentation module, and module_name is the desired name of the instrumentation module.

Note

To determine the architecture notation of a running kernel, run uname -m.

Once the instrumentation module is compiled, copy it to the target system and then load it using:

staprun module_name.ko

For example, to create the instrumentation module simple.ko from a SystemTap script named simple.stp for the target kernel 2.6.32-53.el6, use the following command:

stap -r 2.6.32-53.el6 -e 'probe vfs.read {exit()}' -m simple -p4

This will create a module named simple.ko. To use the instrumentation module simple.ko, copy it to the target system and run the following command (on the target system):

staprun simple.ko

Important

The host system must be the same architecture and running the same distribution of Linux as the target system in order for the built instrumentation module to work.