Bottom Half Handling
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 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
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:
- 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,
- This handler is used to process console messages,
- This handler is used to process tty messages,
- This handler handles general network processing,
- 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
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
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