The author's Chumby 8, a vintage internet appliance, consistently ran at 100% CPU usage due to a kernel bug affecting the way the CPU's clock frequency was handled. The original kernel expected a constant clock speed, but the Chumby's CPU dynamically scaled its frequency. This discrepancy caused the kernel's timekeeping functions to malfunction, leading to a busy loop that consumed all available CPU cycles. Upgrading to a newer kernel, compiled with the correct configuration for a variable clock speed, resolved the issue and brought CPU usage back to normal levels.
This blog post, titled "Why is my CPU usage always 100%? (Upgrading my Chumby 8 kernel part 9)", details the author's ongoing journey to upgrade the Linux kernel on their Chumby 8, a now-discontinued internet appliance. A persistent issue of 100% CPU utilization plagues the device after the kernel upgrade, prompting a deep dive into diagnosing the root cause.
Initially, the author suspects a runaway process is consuming all available CPU cycles. Using the top command, they identify the culprit as the kworker process, specifically a kernel thread dedicated to handling software interrupts. This discovery shifts the focus from a misbehaving user-space application to a problem within the kernel itself.
The author's investigation then explores various potential sources of excessive software interrupts. They meticulously eliminate possibilities such as network interrupts by disconnecting the device from the network, and timer interrupts by analyzing their frequency and confirming they are within expected parameters.
The post highlights the challenges of debugging kernel-level issues, especially on an embedded system with limited resources and debugging tools. The author leverages the available tools, including top, /proc/interrupts, and kernel debugging messages, to progressively narrow down the problem.
Through a process of elimination and careful observation, the author eventually identifies the excessive software interrupts as stemming from the SD card driver. The continuous stream of interrupts from the SD card controller overwhelms the system, leading to the observed 100% CPU usage. While the exact reason for the SD card driver's behavior remains unclear at the end of the post, the author pinpoints the source of the problem and sets the stage for further investigation in future installments. The post concludes by emphasizing the iterative nature of debugging and the importance of systematically eliminating potential causes.
Summary of Comments ( 74 )
https://news.ycombinator.com/item?id=42649862
The Hacker News comments primarily focus on the surprising complexity and challenges involved in the author's quest to upgrade the kernel of a Chumby 8. Several commenters expressed admiration for the author's deep dive into the embedded system's inner workings, with some jokingly comparing it to a software archaeological expedition. There's also discussion about the prevalence of inefficient browser implementations on embedded devices, contributing to high CPU usage. Some suggest alternative approaches, like using a lightweight browser or a different operating system entirely. A few commenters shared their own experiences with similar embedded devices and the difficulties in optimizing their performance. The overall sentiment reflects appreciation for the author's detailed troubleshooting process and the interesting technical insights it provides.
The Hacker News post discussing the blog post "Why is my CPU usage always 100%? Upgrading my Chumby 8 kernel (Part 9)" has several comments exploring various aspects of the situation and offering potential solutions.
One commenter points out the inherent difficulty in debugging such embedded systems, highlighting the lack of sophisticated tools and the often obscure nature of the problems. They sympathize with the author's struggle, acknowledging the frustration that can arise when dealing with limited resources and cryptic error messages.
Another commenter questions the author's decision to stick with the older kernel (2.6.32), suggesting that moving to a more modern kernel might be a more efficient approach in the long run. They acknowledge the author's stated reasons for remaining with the older kernel (familiarity and control) but argue that the benefits of a newer kernel, including potential performance improvements and bug fixes, might outweigh the effort involved in upgrading.
A third commenter focuses on the specific issue of the
kworkerprocess consuming high CPU. They suggest investigating whether a driver is misbehaving or if some background process is stuck in a loop. They propose using tools likestraceorperfto pinpoint the culprit and gain a better understanding of the kernel's behavior. This commenter also mentions the possibility of a hardware issue, although they consider it less likely.Further discussion revolves around the challenges of real-time systems and the potential impact of interrupt handling on CPU usage. One commenter suggests examining interrupt frequencies and considering the possibility of interrupt coalescing to reduce overhead.
Finally, there's a brief exchange about the Chumby device itself, with one commenter expressing nostalgia for the device and another sharing their own experience with embedded systems development. This adds a touch of personal reflection to the technical discussion.
Overall, the comments provide a valuable extension to the blog post, offering diverse perspectives on debugging embedded systems, troubleshooting high CPU usage, and the specific challenges posed by the Chumby 8 and its older kernel. The commenters offer practical suggestions and insights drawn from their own experiences, creating a collaborative problem-solving environment.