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    SubjectTracing/ftrace: trouble with trace_entries and trace_pipe
    Hi Steven and others,

    first a minor bug: it seems the units of /debug/tracing/trace_entries
    is different for read and write. This is confusing for the users, since
    I can't say "if you have problems, double it". If I read from it
    something like 16422, then write back 16422, next I read 214. I can't
    recall the exact numbers, but the difference really is two orders of
    magnitude. I have 1 GB RAM in this box, so that shouldn't be an issue.

    My other problem is with trace_pipe. It is again making 'cat' quit too
    early. The condition triggered is
    if (!tracer_enabled && iter->pos) {
    in tracing_read_pipe(), and it is followed by triggering
    /* stop when tracing is finished */
    if (trace_empty(iter)) {
    and then sret=0, so read returns 0 and 'cat' exits.

    Now, I am trying my mmiotrace marker patches, but as far as I can tell,
    nothing I modified is the reason for this. I didn't yet explicitly test
    for it, though. I'll send these patches after I hear from Frederic.

    The cat-quit problem is not a constant state. After boot, I could play
    with my markers and testmmiotrace without cat quitting. Then something
    happens, and cat starts the quitting behaviour, and won't get to normal
    by disabling and enabling mmiotrace.

    I have a couple of wild guesses of what might be related:
    - ring buffer wrap-around
    - ring buffer overflow (at first try I hit these, the second try
    after putting debug-pr_info's in place I don't hit this)
    - ring buffer resize (after playing with trace_entries, cat-quit
    problem was present, though it might have been present before)

    After viewing the git history, I have some more guesses, mainly
    related to setting tracer_enabled to 0.
    - commit 2b1bce1787700768cbc87c8509851c6f49d252dc
    I don't see where tracer_enabled would be set to 1, when
    mmiotrace is enabled. It used to default to 1 and mmiotrace was happy.
    - __tracing_open() sets it to 0 (not called for the pipe)
    - tracing_release() sets it to 1
    - tracing_ctrl_write() toggles it
    - tracing_read_pipe() tests it
    - tracer_alloc_buffers() uses it
    And other tracers seem to use it a lot.

    Mmiotrace does use the tracer::ctrl_update hook, and allow/disallow
    calls to __trace_mmiotrace_{rw,map}() via enabling/disabling the whole
    mmiotrace core. Is this not enough, or is it inappropriate?

    It seems tracer_enabled is used by the trace framework itself to
    enable/disable... what? Hmm, maybe nothing I care about.

    Should mmiotrace simply do
    tracer_enabled = 1;
    in mmio_trace_init()?

    Should mmiotrace test tracer_enabled, and if so, when?


    Pekka Paalanen

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      Last update: 2008-09-15 21:49    [W:0.025 / U:69.308 seconds]
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