Chapter 39. Getting started with SystemTap
As a system administrator, you can use SystemTap to identify underlying causes of a bug or performance problem on a running RHEL system.
As an application developer, you can use SystemTap to monitor in fine detail how your application behaves within the RHEL system.
39.1. The purpose of SystemTap
SystemTap is a tracing and probing tool that you can use to study and monitor the activities of your operating system (particularly, the kernel) in fine detail. SystemTap provides information similar to the output of tools such as netstat, ps, top, and iostat. However, SystemTap provides more filtering and analysis options for collected information. In SystemTap scripts, you specify the information that SystemTap gathers.
SystemTap aims to supplement the existing suite of Linux monitoring tools by providing users with the infrastructure to track kernel activity and combining this capability with two attributes:
- Flexibility
- the SystemTap framework enables you to develop simple scripts for investigating and monitoring a wide variety of kernel functions, system calls, and other events that occur in kernel space. With this, SystemTap is not so much a tool as it is a system that allows you to develop your own kernel-specific forensic and monitoring tools.
- Ease-of-Use
- SystemTap enables you to monitor kernel activity without having to recompile the kernel or reboot the system.
39.2. Installing SystemTap
To begin using SystemTap, install the required packages. To use SystemTap on more than one kernel where a system has multiple kernels installed, install the corresponding required kernel packages for each kernel version.
Prerequisites
- You have enabled debug repositories as described in Enabling debug and source repositories.
Procedure
Install the required SystemTap packages:
# dnf install systemtap
Install the required kernel packages:
Using
stap-prep:# stap-prep
If
stap-prepdoes not work, install the required kernel packages manually:# dnf install kernel-debuginfo-$(uname -r) kernel-debuginfo-common-$(uname -i)-$(uname -r) kernel-devel-$(uname -r)
$(uname -i)is automatically replaced with the hardware platform of your system and$(uname -r)is automatically replaced with the version of your running kernel.
Verification
If the kernel to be probed with SystemTap is currently in use, test if your installation was successful:
# stap -v -e 'probe kernel.function("vfs_read") {printf("read performed\n"); exit()}'A successful SystemTap deployment results in an 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. 1 read performed 2 Pass 5: run completed in 10usr/40sys/73real ms. 3
The last three lines of output (beginning with
Pass 5) indicate that:
39.3. Privileges to run SystemTap
Running SystemTap scripts requires elevated system privileges but, in some instances, non-privileged users might need to run SystemTap instrumentation on their machine.
To allow users to run SystemTap without root access, add users to both of these user groups:
stapdevMembers of this group can use
stapto run SystemTap scripts, orstaprunto run SystemTap instrumentation modules.Running
stapinvolves compiling SystemTap scripts into kernel modules and loading them into the kernel. This requires elevated privileges to the system, which are granted tostapdevmembers. Unfortunately, such privileges also grant effective root access tostapdevmembers. As such, only grantstapdevgroup membership to users who can be trusted with root access.stapusr-
Members of this group can only use
staprunto run SystemTap instrumentation modules. In addition, they can only run those modules from the/lib/modules/kernel_version/systemtap/directory. This directory must be owned only by the root user, and must only be writable by the root user.
39.4. Running SystemTap scripts
You can run SystemTap scripts from standard input or from a file.
Sample scripts that are distributed with the installation of SystemTap can be found in the Useful examples of SystemTap scripts or in the /usr/share/systemtap/examples directory.
Prerequisites
- SystemTap and the associated required kernel packages are installed as described in Installing Systemtap.
To run SystemTap scripts as a normal user, add the user to the SystemTap groups:
# usermod --append --groups stapdev,stapusr user-name
Procedure
Run the SystemTap script:
From standard input:
# stap -e "probe timer.s(1) {exit()}"This command instructs
stap -eto run the script in parenthesis to standard input.From a file:
# stap file_name.stp
39.5. Useful examples of SystemTap scripts
Sample example scripts that are distributed with the installation of SystemTap can be found in the /usr/share/systemtap/examples directory.
You can use the stap command to execute different SystemTap scripts:
- Tracing function calls
You can use the
para-callgraph.stpSystemTap script to trace function calls and function returns.# stap para-callgraph.stp argument1 argument2The script takes two command-line arguments: The name of the function(s) whose entry/exit you are tracing. An optional trigger function, which enables or disables tracing on a per-thread basis. Tracing in each thread will continue as long as the trigger function has not exited yet.
- Monitoring polling applications
You can use the timeout.stp SystemTap script to identify and monitor which applications are polling. Knowing this, you can track unnecessary or excessive polling, which helps you pinpoint areas for improvement in terms of CPU usage and power savings.
# stap timeout.stpThis script tracks how many times each application uses
poll,select,epoll,itimer,futex,nanosleepandSignalsystem calls over time- Tracking system call volume per process
You can use the
syscalls_by_proc.stpSystemTap script to see what processes are performing the highest volume of system calls. It displays the 20 processes performing the most of system calls.# stap syscalls_by_proc.stp- Tracing functions called in network socket code
You can use the
socket-trace.stpexample SystemTap script to trace functions called from the kernel’s net/socket.c file. This helps you identify how each process interacts with the network at the kernel level in fine detail.# stap socket-trace.stp- Tracking I/O time for each file read or write
You can use the
iotime.stpSystemTap script to monitor the amount of time it takes for each process to read from or write to any file. This helps you to determine what files are slow to load on a system.# stap iotime.stp- Track IRQ’s and other processes stealing cycles from a task
You can use the
cycle_thief.stpSystemTap script to track the amount of time a task is running and the amount of time it is not running. This helps you to identify which processes are stealing cycles from a task.# stap cycle_thief.stp -x pid
You can find more examples and information about SystemTap scripts in the /usr/share/systemtap/examples/index.html file. Open it in a web browser to see a list of all the available scripts and their descriptions.
Additional resources
-
/usr/share/systemtap/examplesdirectory
