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    SubjectRe: Tracing/ftrace: trouble with trace_entries and trace_pipe

    On Mon, 15 Sep 2008, Pekka Paalanen wrote:

    > 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.

    You write to it the same number that you read from and it returned
    something different?? That is indeed a bug, since it should definitely
    detect that. Is this linux-tip? I'll have to play with it to make sure
    nothing broke it recently.

    I just tried the latest mainline, and it seemed to work there.

    > 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?

    No, tracer_enabled is something that is internal to the tracer

    if you read from either tracing/trace or tracing/latency_trace it will
    disable the tracer while you dump. But you should not be doing that. The
    trace_pipe should not disable that either.

    -- Steve

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