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    SubjectRe: [PATCH][2.5] hangcheck-timer
    On Mon, 2003-01-20 at 18:00, Joel Becker wrote:
    > On Mon, Jan 20, 2003 at 05:42:16PM -0800, john stultz wrote:
    > > get_cycles() is a poor method for determining "real time".
    > > Please use do_gettimeofday().
    > Does do_gettimeofday() exist on all platforms? Does it indeed
    > give actual wall clock time, instead of the inaccurate time jiffies can
    > give?

    Yep, do_gettimeofday is called from generic code in sys_gettimeofday()
    (kernel/time.c). It returns the same value userspace code would see
    calling gettimeofday().

    > > > + if (tsc_diff > hangcheck_tsc_margin) {
    > >
    > > but now we're using it to compare cycles! 180sec != 180 cycles
    > Look at the calculations. I'm comparing cycles to cycles,
    > calculated from the original seconds.

    Ah! Ok, I missed the conversion in hangcheck_init. Even so, the default
    initializer is misleading. Yea, that's it... :)

    > > Additionally, this code doesn't take systems that have unsync'ed TSCs,
    > > or systems that change cpu frequency into account. Again, please use
    > > do_gettimeofday(). Then you can then talk about the values returned in
    > > secs and usecs, and I believe things will be much more clear.
    > I'll look into it, but it must absolutely be in terms of wall
    > clock time as measured from outside the system.

    Completely understandable. do_gettimeofday will give you just that (w/o
    the conversion muck w/ HZ and loops_per_jiffy).


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