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    SubjectCONFIG_PRINTK_TIME woes
    It has been pointed out to me that ia64 doesn't boot
    with CONFIG_PRINTK_TIME=y. The issue is the call to
    sched_clock() ... which on ia64 accesses some per-cpu
    data to adjust for possible variations in processor
    speed between different cpus. Since the per-cpu page
    is not set up for the first few printk() calls, we die.

    Is this an issue on any other architecture? Most versions
    of sched_clock() seem to just scale jiffies into nanoseconds,
    so I guess they don't. s390, sparc64, x86 and x86_64 all
    have more sophisticated versions but they don't appear to me
    to have limitations on how early they might be called.

    Possible solutions:

    1) Fix ia64 version of sched_clock() to check whether
    the per-cpu page hase been initialized before using it.

    I don't like this solution as it will make sched_clock()
    slower for its primary uses in kernel/sched.c ... none of
    which can be called before the per-cpu area is initialized.

    2) Add some test to the printk() path along the lines of:

    t = (can_call_sched_clock) ? sched_clock() : 0;

    This is somewhat ugly ... perhaps too unpleasant for words in
    that can_call_sched_clock is a per-cpu concept!

    3) Make the printk() code get the time in some other way.

    Using sched_clock() here seems wrong. The ia64 version
    has a big fat comment about not comparing the results of
    two calls from different cpus ... but since printk() calls
    can come from any cpu ... it seems likely that a user who
    turns on PRINTK_TIME might subtract timestamps from two
    lines to see how long it was between the messages. This
    could have significant error on a system that has been up
    for a long time.

    But I don't have a better suggestion from existing code. I
    assume that sched_clock() was picked as it is both fast and


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      Last update: 2005-08-18 20:49    [W:0.032 / U:5.540 seconds]
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