How do OSes Handle context switching?

Question

As I can understand, every OS need to have some mechanism to periodically check if it should run some tasks and suspend others.

One way would be some kind of timer on whose expiry the OS will check if it should run/suspend some task.

Generally, say on a ARM system that would probably be some kind of ISR.

My real question, is that I've been ABLE to only visualize this and not see it somewhere. Could some one point to some free/open RTOS code where I can actually see the code that handles the preemption/scheduling?

Answer 1

You can take a look at xv6.
Its not an RTOS, it is just a skeleton OS(based on V6 unix) meant for academic purpose.
In the XV6 book take a look at chapter 4, there is explanation along with the code as to how scheduling is done on a small OS like xv6.XV6 puts a process to sleep when it is waiting for disk or some I/O operation, there is also timer interupt every 100msec to switch a process.
There is also explanation with code on how the context switching takes place, what information is saved( context frame of a process), how the switch from user to kernel mode happens when the scheduler has to run.
The best part is that the amount of reading you have to do to understand these concepts is very less unlike some reference book on OS :) The code is relatively small, you can infact run the XV6 on qemu set breakpoints in the sched , swtch and other functions and actually see the information saved during a context switch.(how to run xv6 in this link)
You dont have to read previous chapters to understand the chapter4. There isnt much dependency,xv6 uses struct proc to identify a process, ptable for all the current running process in the system, proc->conext -refers to the state the process is in (register value etc) , this is saved by the scheduler. Cheers :)

Answer 2

freertos.org. The entire OS is open source, and right there for you to see. And there are dozens of different ports to compare and contrast. For the context switch code, you will want to look in the ports directory, in any one of many files called port.c, port.asm, etc. And yes, in the case of freertos all context switches are performed in interrupts (a tick timer ISR, or any other SysCall interrupt).

A context switch is very-much processor specific, as the list of registers to save and the assembly code to save them varies between processor families, and sometimes within a given family. As a result each port has a separate file for this code.

The scheduling (selection of next task to run), on the other hand, is done in a file called tasks.c, which is common to all ports and references the port-specific code.

Answer 3

It is not the case than an RTOS simply context switches periodically - that is how most GPOS work. In an RTOS the scheduler runs on any scheduling event. These include system-tick, but also message post, event trigger, semaphore give, or mutex unlock for example.

On ARM Cortex-M the CMSIS 3.x includes an RTOS API (intended primarily for RTOS developers rather than a complete RTOS itself), the source for this will include a context switching mechanism.

If you want a detailed description for a simple RTOS you might consider reading µC/OS-II: The Real-Time Kernel or the slightly more sophisticated µC/OS-III: The Real-Time Kernel .

FreeRTOS is increasingly popular, though perhaps a little unconventional architecturally. A more complete (in that it is not just a scheduling kernel but a more complete OS) and very powerful option is eCos.