Fixing Flaky Kubernetes Sandbox VCPU Allocation Tests

Hey guys! Let's dive into a common headache we've been seeing with our Kubernetes sandbox tests, specifically the k8s-sandbox-vcpus-allocation.bats script. It's been acting a bit flaky, failing around 5% of the time. This isn't ideal, as consistent test results are super important for maintaining the stability and reliability of our systems. The error message we're seeing often looks something like this:

not ok 1 Check the number vcpus are correctly allocated to the sandbox in 14452ms
# (in test file k8s-sandbox-vcpus-allocation.bats, line 49)
#   `[ "${log}" -eq "${expected_vcpus[$i]}" ]' failed with status 2

This specific failure points to an issue where the actual number of vCPUs allocated to a sandbox doesn't match what the test expects. It’s a bit mysterious because our colleague, Wainer, has tried running this test dozens of times locally and hasn't been able to reproduce it. This kind of intermittent failure is often the trickiest to debug, but that's exactly what we're here to do. We've seen these failures pop up, for instance, on December 3rd, and they're showing up in our CI/CD pipelines too, like this example: https://github.com/kata-containers/kata-containers/actions/runs/19913166970/job/57086950422. Tracking these issues is crucial, so let's break down what might be causing this k8s sandbox vcpus allocation test flaky behavior and how we can nail it down.

Understanding the k8s-sandbox-vcpus-allocation.bats Test

Alright, let's get into the nitty-gritty of what this k8s-sandbox-vcpus-allocation.bats test is actually trying to achieve. At its core, this test is designed to verify that when we create a Kubernetes sandbox (which, in the context of Kata Containers, means creating a lightweight virtual machine that runs your container workloads), the correct number of virtual CPUs (vCPUs) are assigned to it. This is a pretty fundamental check. Think about it: if your sandbox doesn't get the right amount of processing power, your applications running inside might perform poorly, or worse, they might not run at all. The test script uses the bats framework, which is a popular choice for writing shell scripts as tests. It essentially simulates creating a sandbox, then queries the system to see how many vCPUs were actually allocated, and compares that number against an expected value. The line causing the failure, # '[ "${log}" -eq "${expected_vcpus[i]}" ]' failed with status 2`, tells us that the comparison between the logged vCPU count (`{log}`) and the expected vCPU count (`{expected_vcpus[i]}`) failed. This comparison is done using the `-eq` operator, which checks for numerical equality. A status of 2 often indicates a general error in the test execution itself, but in this context, it's stemming from that failed assertion.

Now, why would this test be flaky? Flakiness means the test doesn't fail every single time. It passes most of the time, but then randomly fails. This suggests that the issue isn't a fundamental bug in how vCPUs are allocated all the time, but rather something that happens under specific, perhaps transient, conditions. We're talking about scenarios that aren't easily reproducible on a local machine because they might depend on the specific state of the host system, the timing of other processes, or the overall load on the system where the test is being executed. The fact that Wainer can't reproduce it locally reinforces this idea. It’s not like a simple typo or a wrong configuration that’s always present; it’s more subtle. We need to consider factors like resource contention, timing issues in asynchronous operations, or differences between the test environment and a local development setup. Understanding these underlying mechanisms is key to figuring out why the k8s sandbox vcpus allocation test flaky symptom is appearing, and more importantly, how to make it disappear for good.

Why Is This Flaky k8s sandbox vcpus allocation test flaky Happening?

So, the million-dollar question is: why is this specific k8s sandbox vcpus allocation test flaky? Since it’s not consistently failing, we can rule out a straightforward coding error in the vCPU allocation logic itself. Instead, we need to consider environmental factors and timing. Here are a few prime suspects that often lead to such flaky behavior in container and orchestration environments:

1. Resource Contention and Scheduling: The most common culprit for intermittent test failures is resource contention. When the test runs in a CI/CD environment, there might be other jobs or processes running on the same host that are consuming CPU, memory, or I/O. If the host system is overloaded, the kernel’s scheduler might delay or deprioritize the process responsible for allocating vCPUs to your sandbox. This delay could cause the sandbox creation to take longer than expected, or worse, the vCPU allocation might not complete before the test script checks for it. The test expects a certain number of vCPUs to be ready within a specific timeframe (implied by the 14452ms timeout in the error message), and if the underlying system operations are slower due to contention, the test will fail. It's like trying to grab a seat at a crowded concert – sometimes you get one instantly, other times you have to wait, and maybe the show starts before you even get in.

2. Timing Issues in Asynchronous Operations: Kubernetes and container runtimes often involve a lot of asynchronous operations. When a sandbox is created, multiple steps happen in parallel or in a sequence where one step needs to wait for another to complete. It's possible that the test script is checking the vCPU allocation just before the underlying system has fully finalized it. Even a millisecond’s difference can cause a test to fail. Think of it like checking if a package has arrived before the delivery truck has actually reached your doorstep. The infrastructure might be about to report the vCPUs, but the test is asking too soon. This is especially true in distributed systems where communication delays between different components can add up.

3. Host System Differences: As mentioned, Wainer can't reproduce this locally. This strongly suggests a difference between his local environment and the CI/CD environment where the test is failing. This could be due to different host operating system versions, kernel versions, CPU architectures, or even different configurations of the underlying virtualization technology (like QEMU or KVM). The way vCPUs are exposed and managed by the host kernel can vary, leading to subtle differences in how Kata Containers interacts with it. What works flawlessly on one setup might have edge cases on another.

4. Race Conditions: This is a classic concurrency problem. A race condition occurs when the outcome of a process depends on the unpredictable timing of events. In our case, it’s possible that the test, the sandbox creation process, and some other background system activity are all trying to access or modify the same resources (like CPU affinity settings or process lists) simultaneously. Depending on which operation