问题
Although I have implemented many projects in C, I am completely new to operating systems. I tried real time linux on Discovery board (STM32) and got the correct results for blinking LED but I didn't really understand the whole process since I just followed the steps and could not find whole description for each step on the internet.
I want to implement scheduling on real time linux. What is the best way to start? Any sites, books, tutorials available?
Complete RTLinux process description will be appreciated.
Thanks in adv.
回答1:
The transition from "bare metal" to OS based programming is something that I experienced in reverse. I started out a complete software guy, totally into the OS side of things and over time I have moved to the opposite of that (even designing circuits in VHDL!). My advice would be to start simple. Linux is pretty complex, and everywhere you look there are many layers of things all working together to deliver the final product. If you are dead set on a real time linux extension, I'd be happy to suggest https://xenomai.org/ which is a real time extension for linux.
However, to more specifically address your question about implementing scheduling in Linux, you can, but it will be a large amount of work and can be very complicated. The OS uses a completely fair scheduling process ( http://en.wikipedia.org/wiki/Completely_Fair_Scheduler ) and whenever you spin up a thread, it simply gets added to the list to run. This can differ slightly if you implement your code in kernel space as a driver, rely on hardware interrupts, etc., but in general, this is how Linux works. Real time generally means that it has the ability to assign threads one of several different priorities and utilize thread preemption fully at any given time which are concepts that aren't really a part of vanilla Linux. It has some notion of this, but it has limitations that can cause problems when you are looking for real time behavior from Linux.
What may be helpful to you is an RTOS. If you are looking for a full on Real Time Operating System, check out FreeRTOS http://www.freertos.org/ . It has a large community and supports a lot of different devices out of the box with a large amount of example code. They even support your specific board with an example package, so you can give it a shot with nothing to lose! http://www.freertos.org/FreeRTOS-for-Cortex-M3-STM32-STM32F100-Discovery.html . It gives you access to many OS ish constructs like network APIs, memory management, and threading without the overhead and latency of a huge OS. With an RTOS, you create tasks and assign them priorities so you become the scheduler and are no longer at the mercy of the OS. You run the OS, not the OS runs you (if that makes sense). Plus, the constructs offered within an RTOS will feel much like bare metal code and thus will be much easier to follow, understand, and fully learn. It is a more simple world to learn the base building blocks of a full blown OS such as Linux or Windows. If this option sounds good, I would suggest looking through the supported devices on FreeRTOS website and picking one you would like to experiment with and then go for it. I would highly recommend this as a way to learn about scheduling and OS constructs in general as it is as simple as you can get and open source. Once you have the basics of an RTOS down, buying a book about Linux specifically wouldn't be a bad idea. Although there are many free resources on the web related to learning about Linux, they are commonly contradictory, and can be misleading. Pile on learning Linux specific knowledge along with OS in general, and it can feel overwhelming. Starting simpler will help keep you from getting burnt out and minimize the amount of time you spend feeling lost. Linux is definitely a learning process, but like with any learning process, start simple, keep your ultimate goal in mind, make a plan, and take small, manageable steps along that plan until you look up and find yourself exactly where you want to be. Then go tackle the next mountain!
回答2:
The real-time Linux landscape is quite confusing. 99.99% of the information out there is just plain obsolete.
First, there are lots of "microkernels" that run Linux as one task. (Such as the defunct RTLinux). The problem is that you must write your real-time task to a different API, and can't depend on anything in Linux, because Linux will be frozen in the background while your task runs. So unless your task is dead-simple ("stop the motors when I press this button"), this approach will cause more pain than gain.
Next, there is the realtime Linux patch set. This hasn't been doing so well. because of the next item:
Lastly, the current Linux kernel has gotten rid of the problems that caused people to need realtime in the past. You can even turn off Linux on one of your processors to have full control of the CPU. See also this paper.
To answer your question: I see two different paths you could take:
1) Start with a normal 3.xx Linux kernel and explore the various APIs and realtime techniques (i.e. realtime priorities, memory pinning, etc.) This can get you "close enough" for 99% of what people want "realtime" for. If it's good enough for high frequency trading, it's probably good enough for you.
2) If you have a hard realtime requirement and you are worried that Linux won't cut it, then (as Nick mentioned above), just go buy a processor and write your realtime code with no OS. By splitting up your "realtime" and "non-realtime" code onto different CPUs, you will make the whole system simpler and much more robust.
回答3:
If you want to learn real-time operating systems then I suggest that you get an FPGA, for example the Altera DE2, and experiment with your own operating system and ucos. You can read a good text about embedded RTOS here.
You could also get a Linux Raspberry and write your own operating system for that.
来源：https://stackoverflow.com/questions/29358214/what-is-the-best-way-to-start-programming-with-real-time-linux