Figure: Bottom Half Handling Data Structures

There are often times when you don’t want the kernel to do any work at all. A good example of this is during interrupt processing. When the interrupt was asserted, the processor stopped what it was doing and the operating system delivered the interrupt to the appropriate device driver. Device drivers should not spend too much time handling interrupts as, during this time, nothing else in the system can run. There is often some work that could just as well be done later on. Linux’s bottom half handlers were invented so that device drivers and other parts of the Linux kernel could queue work to be done later on. The figure above shows the kernel data structures associated with bottom half handling.

There can be up to 32 different bottom half handlers, which are referenced through a vector of pointers called bh_base. These pointers point to each of the kernel’s bottom half handling routines. bh_active and bh_mask have their bits set according to what handlers have been installed and are active. If bit N of bh_mask is set then the Nth element of bh_base contains the address of a bottom half routine. If bit N of bh_active is set then the Nth bottom half handler routine should be called as soon as the scheduler deems reasonable. These indices are statically defined. The timer bottom half handler (index 0) is the highest priority, the console bottom half handler (index 1) is next in priority and so on. Typically the bottom half handling routines have lists of tasks associated with them. For example, the immediate bottom half handler works its way through the immediate tasks queue (tq_immediate), which contains tasks that need to be performed immediately.

Some of the kernel‘s bottom half handers are device specific, but others are more generic:

TIMER

This handler is marked as active each time the system’s periodic timer interrupts and is used to drive the kernel‘s timer queue mechanisms,

CONSOLE

This handler is used to process console messages,

TQUEUE

This handler is used to process tty messages,

NET

This handler handles general network processing,

IMMEDIATE

This is a generic handler used by several device drivers to queue work to be done later.

Whenever a device driver, or some other part of the kernel, needs to schedule work to be done later, it adds work to the appropriate system queue, for example the timer queue, and then signals the kernel that some bottom half handling needs to be done. It does this by setting the appropriate bit in bh_active. Bit 8 is set if the driver has queued something on the immediate queue and wishes the immediate bottom half handler to run and process it. The bh_active bitmask is checked at the end of each system call, just before control is returned to the calling process. If it has any bits set, the bottom half handler routines that are active are called. Bit 0 is checked first, then 1 and so on until bit 31.

The bit in bh_active is cleared as each bottom half handling routine is called. bh_active is transient; it only has meaning between calls to the scheduler and is a way of not calling bottom half handling routines when there is no work for them to do.