Why do we need Interrupt context?

Question

I am having doubts, why exactly we need interrupt context? Everything tells what are the properties but no one explains why we come up with this concept?

Another doubt related to same concept is, If we are not disabling the interrupt in interrupt handler, then what is the use of running this interrupt handler code in interrupt context ?

Answer 1

Why do we need Interrupt context?

First, what do we mean by interrupt context? A context is usually a state. There are two separate concepts of state.

CPU context

Every CPU architecture has a mechanism for handling interrupts. There maybe a single interrupt vector called for every system interrupt, or the CPU/hardware may be capable of dispatching the CPU to a particular address based on the interrupt source. There are also mechanisms for masking/unmasking interrupts. Each interrupt maybe masked individually, or there maybe a global mask for the entire CPU(s). Finally, there is an actual CPU state. Some may have separate stacks, register sets, and CPU modes implying some memory and other privileges. Your question is about Linux in general and it must handle all cases.

Linux context

Generally all of the architectures have a separate kernel stack, process context (ala ps) and VM (virtual memory) context for each process. The VM has different privileges for user and kernel modes. In order for the kernel to run all the time, it must remain mapped for all processes on a device. A kernel thread is a special case that doesn't care so much about the VM, because it is privileged and can access all kernel memory. However, it does have a separate stack and process context. User registers are typically stored upon the kernel stack when exceptions happen. Exceptions are at least page faults, system calls and interrupts. These items may nest. Ie, you may call write() from user space and while the kernel is transferring a user buffer, it may page fault to read some swapped out user space data. The page fault may again have to service an interrupt.

Interrupt recursion

Linux general wants you to leave interrupts masked as the VM, the execptions, and process management (context and context switching) have to work together. In order to keep things simple for the VM, the kernel stack and process context are generally rooted in either a single 4k (or 8k) area which is a single VM page. This page is always mapped. Typically, all CPUs will switch from interrupt mode to system mode when servicing an interrupt and use the same kernel stack as all other exceptions. The stack is small so to allow recursion (and large stack allocation) can blow up the stack resulting in stack overflows at the kernel level. This is bad.

Atomicity

Many kernel structures need to stay consistent over multiple bus cycles; Ie, a linked list must update both prev and next node links when adding an element. A typical mechanism to do this maybe to mask interrupts, to ensure the code is atomic. Some CPUs may allow bus locking, but this is not universal. The context switching code must also be atomic. A consequence of an interrupt is typically rescheduling. Ie, a kernel interrupt handler may have acked a disk controller and started a write operation. Then a kernel thread may schedule to write more buffered data from the original user space write().

Interrupts occurring at any time can break some sub-sytem's assumptions of atomic behavior. Instead of allowing interrupt to use the sub-system, they are prohibited from using it.

Summary

Linux must handle three thing. The current process execution context, the current virtual memory layout and hardware requests. They all need to work together. As the interrupts may happen at any time, they occur in any process context. Using sleep(), etc in an interrupt would put random processes to sleep. Allowing large stack allocation in an interrupt could blow up the limited stack. These design choices limit what can happen in a Linux interrupt handler. Various configuration options can allow re-entrant interrupts, but this is often CPU specific.

A benefit of keeping the top half, now the main interrupt handler small is that interrupt latency is reduced. Busy work should be done in a kernel thread. An interrupt service routine that would need to un-mask interrupts is already somewhat anti-social to the Linux eco-system. That work should be put in a kernel thread.

The Linux interrupt context really doesn't exist in some sense. It is only a CPU interrupt which may happen in any process context. The Linux interrupt context is actually a set of coding limitations that happen as a consequence of this.

Answer 2

The interrupt context is fundamentally different from the process context:

1. It is not associated with a process; a specific process does not serve interrupts, the kernel does. Even if a process will be interrupted, it has no significance over any parameters of the interrupt itself or the routine that will serve it. It follows that at the very least, interrupt context must be different from process context conceptually.

2. Additionally, if an interrupt were to be serviced in a process context, and (re-) scheduled some work at a later time, what context would that run in? The original process may not even exist at that later time. Ergo, we need some context which is independent from processes for a practical reason.

3. Interrupt handling must be fast; your interrupt handler has interrupted (d'oh) some other code. Significant work should be pushed outside the interrupt handler, onto the "bottom half". It is unacceptable to block a process for work which is not even remotely its concern, either in user or in kernel space.

4. Disabling the interrupt is something you can (actually could, before 2.6.36) request to be disabled when registering your ISR. Recall that a handler can serve interrupts on multiple CPUs simultaneously, and can thus race with itself. Non-Maskable Interrupts (NMIs) can not be disabled.