In this lesson, you will learn how to measure system call latency to identify how complex security policies, such as those found in SELinux or AppArmor, impact your application's performance.
The Cost of Policy Enforcement
Every time a process performs a system call—like opening a file or binding a network socket—the kernel must pause to verify if the action is permitted by your loaded security policy. As your policy grows in complexity (adding hundreds of rules, complex transitions, or deep nesting), the "look-up time" required for the kernel to traverse your policy database increases. In high-frequency environments, this overhead can manifest as micro-stutters or increased latency.
Benchmarking with strace
To identify if your security policy is creating a bottleneck, you need to measure the time spent inside the kernel for common system calls. The strace utility is excellent for this. By using the -T flag, you can display the time spent in each system call:
# Trace a process and show time spent in system calls
strace -c -T -p <PID>
When you run this, look at the usecs/call column. If you notice specific system calls like openat or connect taking significantly longer than expected, it is often an indicator that the kernel is spending extra cycles traversing a massive, fragmented security policy to resolve the security context.
Identifying the Bottleneck
If you suspect policy complexity is the culprit, compare your latency metrics in two states:
- Enforcing Mode: With your complex policy loaded.
- Permissive/Disabled Mode: Temporarily relaxed to see if the
usecs/callvalues drop.
If the latency drops significantly in Permissive mode, your policy likely contains redundant rules or inefficient structures that are forcing the kernel to perform exhaustive checks. You can then use tools you’ve learned previously, such as audit2why or modular policy auditing, to prune unnecessary rules and flatten your security logic.
Key Takeaway: Measuring system call latency with tools like strace allows you to quantify the hidden tax that complex security policies impose on kernel operations, helping you balance robust protection with raw system performance.