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    SubjectRe: [BUG -rt] -rt16 hang w/ realtime thread test
    On Sat, 2006-02-11 at 10:34 -0500, Steven Rostedt wrote:
    > On Fri, 10 Feb 2006, john stultz wrote:
    > > Hey Ingo,
    > > I've been hunting a report that lower priority realtime threads are not
    > > preempting higher priority realtime threads. However, in generating test
    > > cases, I found I was locking the system quite frequently.
    > >
    > > The attached test runs to completion on 2.6.15, but with 2.6.15-rt16, it
    > > hangs the box. It could very well be a test issue, but I'm not sure I
    > > see where the problem is.
    > >
    > Hi John,
    > Have you turned on nmi_watchdog and softlockup detect? Just so we can see
    > where it is hung.

    Ugh. Ok, I think I've found the issue.

    The systems I'm testing w/ all use the ACPI PM timer for the
    clocksource. On a whim I forced the TSC to be used and the hang went

    It appears the issue is that the ACPI PM wraps every ~5 seconds. Since
    the test takes longer the 5 seconds to run, the
    timeofday_periodic_hook() function gets starved and we never accumulate
    time. Then as the counter wraps, time starts wrapping thus timers do not
    expire and the test never completes, effectively hanging the box.

    I believe this issue was hit before back when cycle_t was 32bits long,
    thus causing the TSC to wrap every ~4seconds on a 1Ghz box.

    So whats the solution here? Do we need to do something to
    timeofday_periodic_hook is guaranteed to run with some frequency? Or is
    the test just bunk because realtime threads must give up the cpu in
    order for the kernel to function?


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