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    SubjectRe: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15

    * Mark_H_Johnson@raytheon.com <Mark_H_Johnson@raytheon.com> wrote:

    > [...] I also had several cases where I "triggered" a trace but no
    > output - I assume those are related symptoms. For example:
    >
    > # ./cpu_delay 0.000100
    > Delay limit set to 0.00010000 seconds
    > calibrating loop ....
    > time diff= 0.504598 or 396354830 loops/sec.
    > Trace activated with 0.000100 second delay.
    > Trace triggered with 0.000102 second delay. [not recorded]
    > Trace activated with 0.000100 second delay.
    > Trace triggered with 0.000164 second delay. [not recorded]

    is the userspace delay measurement nested inside the kernel-based
    method? I.e. is it something like:

    gettimeofday(0,1);
    timestamp1 = cycles();

    ... loop some ...

    timestamp2 = cycles();
    gettimeofday(0,0);

    and do you get 'unreported' latencies in such a case too? If yes then
    that would indeed indicate a tracer bug. But if the measurement is done
    like this:

    gettimeofday(0,1);
    timestamp1 = cycles();

    ... loop some ...

    gettimeofday(0,0); // [1]
    timestamp2 = cycles(); // [2]

    then a delay could get inbetween [1] and [2].

    OTOH if the 'loop some' time is long enough then the [1]-[2] window is
    too small to be significant statistically, while your logs show a near
    50% 'miss rate'.

    Ingo
    -
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