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    SubjectRe: [tracing, hang] dumping events gets stuck in synchronise_sched
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    On Wed, 2010-08-18 at 10:55 +1000, Dave Chinner wrote:
    > On Tue, Aug 17, 2010 at 07:07:11PM -0400, Steven Rostedt wrote:

    > > You mean because it uses synchronize_sched() it should be removed? Seems
    > > that it was another kernel bug that caused the issue.
    >
    > Right now I'm using the trace file for exactly what the docuemntation
    > says it should be used for, but it hangs. You're saying that the
    > interface is effectively redundant because there's a trace_pipe
    > interface that shouldn't hang and I should use that instead.

    trace_pipe consumes the data as trace does not.

    >
    > Debug interfaces should be reliable in the face of common problems -
    > a system burning a CPU in a tight loop is a pretty common problem.
    > That leads to three options: either fix the hang, document the fact
    > that the trace file is not reliable and that you should use other
    > interfaces, or remove the interface altogether.

    I could update the documentation to warn that the trace file uses the
    common utility "synchronize_sched()" and that if you are debugging a
    system that has a CPU out of commission, not to use it because,
    synchronize_sched() will never return. If you need to debug this case,
    then use trace_pipe which will consume all data, but is made for live
    reads and does not use synchronize_sched().

    But the tracer is used for more than just debugging. I use it for
    analyzing system behavior, and not to debug anything, which I use the
    trace file for all the time. I usually enable tracing, disable it, then
    look at the trace file. For any live tracing, trace_pipe should be used.
    If I had not documented that, I should go back and do so.

    -- Steve




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