The work done on real-time Linux has benefitted the open-source OS for years, but it was only this week that Linus Torvalds admitted its last piece into the mainline kernel. Exactly what took so long?

After 20 years, Real-Time Linux (PREEMPT_RT) is finally – finally – in the mainline kernel. Linus Torvalds blessed the code while he was at Open Source Summit Europe. […] The real-time Linux code is now baked into all Linux distros as of the forthcoming Linux 6.12 kernel. This means Linux will soon start appearing in more mission-critical devices and industrial hardware. But it took its sweet time getting here. An RTOS is a specialized operating system designed to handle time-critical tasks with precision and reliability. Unlike general-purpose operating systems like Windows or macOS, an RTOS is built to respond to events and process data within strict time constraints, often measured in milliseconds or microseconds. As Steven Rostedt, a prominent real-time Linux developer and Google engineer, put it, “Real-time is the fastest worst-case scenario.” He means that the essential characteristic of an RTOS is its deterministic behavior. An RTOS guarantees that critical tasks will be completed within specified deadlines. […]

So, why is Real-Time Linux only now completely blessed in the kernel? “We actually would not push something up unless we thought it was ready,” Rostedt explained. “Almost everything was usually rewritten at least three times before it went into mainline because we had such a high bar for what would go in.” In addition, the path to the mainline wasn’t just about technical challenges. Politics and perception also played a role. “In the beginning, we couldn’t even mention real-time,” Rostedt recalled. “Everyone said, ‘Oh, we don’t care about real-time.’” Another problem was money. For many years funding for real-time Linux was erratic. In 2015, the Linux Foundation established the Real-Time Linux (RTL) collaborative project to coordinate efforts around mainlining PREEMPT_RT.

The final hurdle for full integration was reworking the kernel’s print_k function, a critical debugging tool dating back to 1991. Torvalds was particularly protective of print_k --He wrote the original code and still uses it for debugging. However, print_k also puts a hard delay in a Linux program whenever it’s called. That kind of slowdown is unacceptable in real-time systems. Rostedt explained: “Print_k has a thousand hacks to handle a thousand different situations. Whenever we modified print_k to do something, it would break one of these cases. The thing about print_k that’s great about debugging is you can know exactly where you were when a process crashed. When I would be hammering the system really, really hard, and the latency was mostly around maybe 30 microseconds, and then suddenly it would jump to five milliseconds.” That delay was the print_k message. After much work, many heated discussions, and several rejected proposals, a compromise was reached earlier this year. Torvalds is happy, the real-time Linux developers are happy, print_K users are happy, and, at long last, real-time Linux is real.

  • @RustyWizard
    2•6 hours ago

    What’s preventing that from working now? If it’s indeterminate latency, then yeah, absolutely. Theoretically this will give you the ability to have a very deterministic loop around the accelerometer data, but 3d printers don’t move all that fast to begin with so having unbounded latency might not matter. The determinism we’re talking about here is on the order of tens of microseconds or less.

    • @[email protected]
      2•6 hours ago

      3D printers can move very fast. They typically don’t because it causes all kinds of deformations in the print. Mostly the issues are in acceleration, decelaration, cornering, and controlling the heat snd flow of melting filament.

      I don’t know whether or not the accelerometer thing can be done in real time, or if there would be any benefit.

      Check out the 2-minute Benchy for an example of how fast a 3D printer can get. This is a test print that typically should take about 45 minutes to an hour at very basic settings.

      But also note the quality of the end product. It looks pretty awful. If we could print accurately at even remotely similar speeds, it would be fantastic.

      • @RustyWizard
        2•5 hours ago

        “Very fast” is relative. 1200mm/s is very fast for 3D printing, no argument. But it’s 1.2mm/millisecond, and we’re talking about time scales in the microsecond range. I suspect you’re going to run into materials issues far before real time performance becomes a limiting factor in print speed and quality.