My Tech notes: Bottom half and techniques to implement it
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Wednesday, June 25, 2008

Bottom half and techniques to implement it

Bottom half and techniques to implement

Bottom half" is a generic operating system term referring to the deferred portion of interrupt processing, so named because it represents the second, or bottom, half of the interrupt processing solution. In Linux, the term currently has this meaning, too. All the kernel's mechanisms for deferring work are "bottom halves." Some people also confusingly call all bottom halves "softirqs," but they are just being annoying. "Bottom half" also refers to the original deferred work mechanism in Linux. This mechanism is also known as a "BH," so we call it by that name now and leave the former as a generic description. The BH mechanism was deprecated a while back and fully removed in 2.5. Currently, there are three methods for deferring work: softirqs, tasklets, and work queues. Tasklets are built on softirqs and work queues are entirely different.

Using Softirqs
Softirqs are reserved for the most timing-critical and important bottom-half processing on the system. Currently only two subsystems networking and SCSIdirectly use softirqs. Additionally, kernel timers and tasklets are built on top of softirqs. If you are adding a new softirq, you normally want to ask yourself why using a tasklet is insufficient. Tasklets are dynamically created and are simpler to use because of their weaker locking requirements, and they still perform quite well. Nonetheless, for timing-critical applications that are able to do their own locking in an efficient way, softirqs might be the correct solution.

Tasklets
Tasklets are a bottom-half mechanism built on top of softirqs. As already mentioned, they have nothing to do with tasks. Tasklets are similar in nature and work in a similar manner to softirqs; however, they have a simpler interface and relaxed locking rules. The decision between whether to use softirqs versus tasklets is simple: You usually want to use tasklets. Softirqs are required only for very high-frequency and highly threaded uses. Tasklets, on the other hand, see much greater use. Tasklets work just fine for the vast majority of cases and they are very easy to use.

In most cases, tasklets are the preferred mechanism with which to implement your bottom half for a normal hardware device. Tasklets are dynamically created, easy to use, and very quick. Moreover, although their name is mind-numbingly confusing, it grows on you: It is cute.

Work Queues
Work queues are a different form of deferring work from what we have looked at so far. Work queues defer work into a kernel thread this bottom half always runs in process context. Thus, code deferred to a work queue has all the usual benefits of process context. Most importantly, work queues are schedulable and can therefore sleep.

Normally, it is easy to decide between using work queues and softirqs/tasklets. If the deferred work needs to sleep, work queues are used. If the deferred work need not sleep, softirqs or tasklets are used.

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