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    SubjectRe: [patch] Real-Time Preemption, -RT-2.6.11-rc2-V0.7.36-04

    * Lee Revell <> wrote:

    > OK. So for application triggered tracing you need LATENCY_TRACING
    > enabled, as described here:

    correct, that too should still work fine - with the small change that
    there's now a separate flag to active it:

    echo 1 > /proc/sys/kernel/trace_user_triggered # default: 0

    it is an orthogonal mechanism to atomicity-debugging.

    since i wrote the above mail 3 months ago, a number of improvements have
    been done to the tracer. There are a handful of modifier feature-flags
    to the tracer, which can be used to get additional functionality. Here's
    a quick summary:

    echo 1 > /proc/sys/kernel/trace_freerunning # default: 0

    will get a 'freerunning' tracer which never stops (and overwrites the
    oldest entries if the trace gets full). Especially with long latencies
    this in some cases can be more informative.

    this flag:

    echo 0 > /proc/sys/kernel/mcount_enabled # default: 1

    causes the tracer to record only key kernel events (schedule/wakeup
    events, etc.), not every kernel function call. This might be useful if
    you want to see the bigger picture and want to validate scheduling logic
    on a bigger scale, spanning a much longer timeframe.

    and if you have stability problems, this flag might be handy:

    echo 1 > /proc/sys/kernel/trace_freerunning # default: 0

    it will dump the current kernel trace to the kernel console if a crash
    happens - obviously only useful with a serial console or netconsole.
    It's a big dump but can make some bugs much easier to debug.

    on SMP:

    echo 1 > /proc/sys/kernel/trace_all_cpus # default: 0

    this flag will cause all activity from all CPUs to be included in the
    trace. This can be useful if it is suspected that a particular latency
    was caused not by the CPU where the latency triggers, but by some other

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      Last update: 2005-03-22 14:10    [W:0.130 / U:7.500 seconds]
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