Updating callout_reset

From: Mike Silbersack (silby_at_silby.com)
Date: 01/26/04

  • Next message: Poul-Henning Kamp: "Re: Updating callout_reset" 
    Date: Sun, 25 Jan 2004 23:29:33 -0600 (CST)
To: arch@freebsd.org

    Based on the work I've been doing with the rtc driver and some issues in
    the tcp stack with high hz values, I'd like to propose a change in the
    callout_reset interface. Currently, the timeout value is specified in
    ticks. This creates two problems:

    #1 - At high hz rates, the maximum timeout possible is reduced; we have
    at least one example of this causing problems in the kernel at present
    (16-bit sbtimeout values getting reduced from 327 to 32 seconds), there
    could be others we haven't caught. Many of these are likely to result
    from integer math errors introduced by authors who hadn't considered
    larger hz values occuring.

    #2 - Using ticks reduces the potential accuracy of wakeups with our
    default hz setting. For example, if one wishes to sleep for 5ms when
    hz=100, the only choices is to request one tick, or 10ms. However, if 5ms
    could be specified, the callout subsystem would be able to schedule more
    precise wakeups in places where the next timer interval was between 5 and
    10ms away.

    Case #1 also provides a second justification for a change; any piece of
    code which requests a timeout must be aware of the system hz; moving to a
    standard time format would create more straightforward code.

    In order to ensure that modules and the like do not break, I would like to
    propose that we leave callout_reset defined as is and create a new
    function, callout_set (or some other clever name) which takes a struct
    *timespec in place of ticks.

    Optimizations as suggested in example #2 above would not be implemented
    right away, but would be possible due to the better interface.

    Thoughts?

    Mike "Silby" Silbersack
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