Thursday, April 29, 2010

LDD3 notes: Passage of time

My notes while reading Linux Device Drivers 3rd edition


  • get_jiffies_64()
  • comparison macros: time_after/before
  • jiffies <-> timeval / timespec
  • get_cycles()

short busy waits

  • n/u/mdelay()

1ms/1s resolution sleeps

  • msleep(), ssleep()

Working with 1/HZ resolution sleeps

  • wait_event_timeout() - returns timeout left, never negative
  • set_current_state(interruptible), schedule_timeout()
  • in_atomic(), in_interrupt()

Sleeping with waitqueues

  • macro: one init_waitqueue_head, multiple wait_event(_interruptible), one wake_up to wake up them all.
  • manual wait_event alternative: prepare_to_wait(), schedule(), finish_wait(), signal_pending()
  • 'exclusive' sleepers are woken up as specified batches or individually, they won't behave like a herd
  • going to schedule soon, use wake_up_onterruptible_sync, it won't reschedule right away
  • never use sleep_on, it's broken w/ race condition


  • atomic context in timer interrupt handler or softirq
  • multiple schedules result in single execution
  • cannot assume process context or access user space memory


  • queue has a kernel process, can sleep, but cannot access user space
  • each work_struct in queue only once at a time
  • can be delayed for jiffies

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