What is the difference between tasklet and workqueue

 

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

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

    • 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.

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