What is the difference between tasklet and workqueue

Question

I am a Linux device driver newbie, and want to know the exact differences between tasklet and workqueue. I have the following doubts:

1. Which kernel stack do interrupts, tasklet and workqueue use when running in interrupt/process context?
2. At what priority would tasklet and workqueue run and can we modify it's priority?
3. If I implement my own work queue list, can I schedule/prioritize it independently?

Answer 1

Tasklets:

• are old (around 2.3 I believe)
• have a straightforward, simple API
• are designed for low latency
• cannot sleep (run atomically in soft IRQ context and are guaranteed to never run on more than one CPU of a given processor, for a given tasklet)

Work queues:

• are more recent (introduced in 2.5)
• have a flexible API (more options/flags supported)
• are designed for higher latency
• can sleep

Bottom line is: use tasklets for high priority, low latency atomic tasks that must still execute outside the hard IRQ context.

You can control some level of priority with tasklets using tasklet_hi_enable/tasklet_hi_schedule (instead of their respective no-_hi versions). From this IBM page:

The normal-priority schedule is performed through the TASKLET_SOFTIRQ-level softirq, where high priority is through the HI_SOFTIRQ-level softirq.

...

Tasklets from the high-priority vector are serviced first, followed by those on the normal vector. Note that each CPU maintains its own normal and high-priority softirq vectors.

With work queues, when creating one, you will use alloc_workqueue (create_workqueue is deprecated) and can pass a flag to ask for higher priority:

WQ_HIGHPRI:

Work items of a highpri wq are queued to the highpri thread-pool of the target gcwq. Highpri thread-pools are served by worker threads with elevated nice level.

Note that normal and highpri thread-pools don't interact with each other. Each maintain its separate pool of workers and implements concurrency management among its workers.

I cannot answer all your questions, but I hope this helps anyway.