Linux Kernel Code Execution Contexts

4
votes

When a process executing in the user space issues a system call or triggers an exception, it enters into the kernel space and kernel starts executing on behalf of the process. Kernel is said to be executing in the process context. Similarly when an interrupt occurs kernel executes in the interrupt context. I have studied about kernel execution in kernel thread, where kernel processes runs in the background.

My Questions are :

  1. Does the kernel execute in any other contexts?

  2. Suppose a process in the user space never executes a system call or triggers an exception or no interrupt occurs, does the kernel code ever execute ?

2
processlinux-kernel
I don't think there can exist a process which never executes a system call. – balki
Agreed. I suppose then only the contexts i mentioned in my question exists where the kernel code runs ? – nitin_cherian
@balki: for(;;) __asm__ __volatile__("nop"); – ninjalj
There is also NMI context, but you can pretty much ignore it. – ninjalj

2 Answers

2
votes

The kernel runs periodically, it sets a timer to fire an interrupt at some predefined frequency (100 Hz (Linux 2.4/x86), 1000Hz (early Linux 2.6/x86), 250Hz (newer Linux 2.6/x86)).

The kernel need to do this in order to do preemptive multitasking. OTOH, OSes only doing cooperative multitasking (Windows 3.1, classic Mac OS) needn't do this, and only switch tasks on response to some call from the running task (which could lead to runaway tasks hanging the whole system).

Note that there is some effort to optimize the use of this timer: newer Linux is smarter when there are no runnable tasks, it sets the timer as far in the future as it can, to allow the CPU to sleep longer and deeper, and preserve power (the CONFIG_NOHZ kernel config option). Running powertop will show the number of wakeups per second, which on an idle system can be much lower than the 250 wakeups per second you'd expect of a traditional implementation.

0
votes

Suppose a process in the user space never executes a system call or triggers an exception or no interrupt occurs, does the kernel code ever execute ?

Assume you have a process p that is running the following code: while(1);. This code will never call into the kernel and won't cause any faults. (It might have set an alarm(3) earlier, causing a signal to be delivered in the future, or it might exceed the setrlimit(2) CPU limit, in which cases the kernel will deliver a signal to the process.)

Or, if another process sends p a signal via kill(2), the kernel will deliver that signal to the process as well.

The signal delivery will either cause a signal handler to run, do nothing (if the signal is ignored or masked), or take the default signal action (which might be nothing or termination).

And, of course, the process execution can be interrupted so the processor can handle interrupts; or a higher-priority process can preempt it.