20 years later, real-time Linux makes it to the kernel - really
20 years later, real-time Linux makes it to the kernel - really
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.
This is super interesting. I’ll admit I wasn’t even aware of this effort. Even real-time usage of Windows relies on a parallel kernel.
This sounds like it’ll create a lot of cool opportunities and reduce friction.
But the RT patches have been available for 20 years... not sure why the fact that it is mainlined would suddenly expand its popularity? It might be easier to get started sure, but people doing RT were already going to such troubles anyway.
It’s like saying GPS was available for decades before, why would putting it in everyone’s phone expand its popularity.
For myself, I’m hoping the nerds and hackers that otherwise found it not worth the effort will start creating tools to manage real time better and start building them into the applications they write. That way you don’t need to pay an arm and a leg to RedHawk for the privilege of dynamically isolating CPUs or have to reboot the computer to modify kernel arguments a la RedHat MRG.
I said it’ll reduce friction, you said it might be easier. Looks like we’re in complete agreement, right?
Music Makers will love this
I'm curious about this. I've started playing with Reaper and getting into music recording and production. I'm very fresh on the scene and haven't used any DAWs on any other OS, except viewed protools on Mac. I can't quite get latency free playback, which may just be user error and configs.
Do you know if something like this will have default benefits out of the box, or will we need to somehow configure our apps and services to utilize these changes? I'm completely ignorant but am really intrigued.
I have a relative who I helped set up RTLinux (?) on Kubuntu back in 2018. I think they were playing around with Helm, Calf, Ardour, Hydrogen, Rosegarden et all and JACK of course, but that's all I remember on the music production side - I was helping more along the lines of PPAs, compiling and configuring and mostly on WE after I came back from somewhere, so 3AM or smth (not good for memory).
Does this mean anything to the average user, or is this a very specific use case?
Probably some use cases for "regular" users. Someone mentioned music production, though that's probably more professional than hobby.
To my understanding, you mostly need real time performance for specialty cases where timing is absolutely critical. So I guess if you were building custom drones or custom control boards for drones, you could use real time Linux for that now since the timing could be guaranteed.
So what about 3D printing? Currently, input shaping uses an accelerometer to calculate resonances and uses that data to adjust movement and reduce flaws in the printing process. For anyone with knowledge of both fields, would this allow a built-in or add-on accelerometer to be used in real time to compensate for momentum and resonances even further?
Pretty specific use case. A normal OS handleds time slicing and core assignment for processes and uses it's judgement for that. So at any time your process can be suspended and you don't know when you get your next time slice.
Same with when you make wait calls. You might say wait 100ms but it may be much longer before your process gets to run again.
In a real time OS if you have real time priority the OS will suspend anything else including it self to give you the time you request. It also won't suspend you no matter how long you use the core.
So if you need to control a process with extreme precision like a chemical manufacturing process, medical device, or flying a rocket where being 10ms late means failure they are required.
However with great power comes great responsibility. You need to make sure your code calls sleep frequently enough that other tasks have time to run. Including things like file io or the gui.
My guess is not a whole lot to the average user, but it would allow for things to still respond when other things have bogged down resources. I am assuming real world applications would be industry like a machine safery stop should always have a quick turn around, and not be delayed by harddrive writes. But may like how they write special OS code for spacecraft where sending and receiving instructions on board has special states and if response isn't given in timely manner the system can recognize, so malfunctions are prevented. There was an artivle/podcast somewhere abouy how this all had to work in realtime and not be queued waiting
No, but you might notice programs get more responsive as time goes on, ofc if people use it wrong you might see more freezes and system crashes.
Heck yeah. Now a microkernel version of Linux would be the icing on top!
after much work, many heated discussions, and several rejected proposals, a compromise was reached earlier this year.
So what was the compromise with print_k??
Doesn't say, but I am curious. They said their workarounds broke other workarounds which caused a lot of implementation delay, but I'm not sure what the actual compromise was to address all that.
Answer probably lies somewhere in the kernel maintainer's mailing list, I'd imagine. Just not equipped to search for it right at the moment.
after using gentoo for a couple months, i think i know a thing or two about spending Real Time getting the linux kernel
Intriguing. The first and last time I saw anything RT kernel related was nearly a decade ago, when I stumbled on it as an alternate kernel in the old synaptic package manager.
This has got to be a good thing for emulation, forwarding api calls can happen in real time now, that would make it entirely more responsive and might be indistinguishable from running on bare metal.
Cool. I remember having to use Red Hawk Linux years ago. It's a real time variant of Red Hat. It was such a pain to get the dependencies workin on it.
Anyone know what real time means here? Does it mean that sleeping a thread is more accurate (as in the thread is resumed at the correct time after calling sleep)? Or is there an API that implements some functionality for something that should run in real time?
I'm not an expert, but have used a real time kernel for scientific research, using rtxi. My understanding is that the real time threads allows the computations to occur in a deterministic amount of time. This is necessary if you want to quickly respond to changes in personal membrane voltage with injections of current, and don't want it to sometimes take longer to calculate how much current to inject.
RT systems are all about responsibility.
When you're driving a vehicle, the vehicle has a lot of CPUs inside that help the steering.
When these CPUs pause - for even 0.4 seconds - something bad can happen. Somebody can get seriously injured, or even killed. So it is very important that these systems never freeze or pause for any reason. That is what RT is all about.