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    SubjectRe: [RFC patch 00/41] LTTng 0.105 core for Linux 2.6.27-rc9

    * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

    > Hi,
    >
    > I spent the last 4-5 months working with the Fujitsu team at
    > implementing the tracer elements identified as goals at Kernel
    > Summit 2008 and at the following Plumber Conference. My idea
    > was to incremententally adapt the LTTng tracer, currently used
    > in the industry and well tested, to those requirements.
    >
    > I spent the last days rearranging/folding/inspecting the LTTng
    > patchset to prepare it for an LKML post. The version 0.105 in
    > the LTTng git tree corresponds to the patchset I am posting
    > here. The said patchset will only include the core features of
    > LTTng, excluding the timestamping infrastructure (trace clock)
    > and excluding the instrumentation.

    I'd like to merge the good bits into the tracing tree. Looking
    at the patches you submitted there's a lot of avoidable overlap
    with existing tracing features either present upstream already
    or queued up for v2.6.30 - and we need to work more on
    eliminating that overlap.

    I dont think there's much fundamental disagreement just
    different implementations - so we should evaluate each of those
    details one by one, iteratively.

    The first step would be to split the patches up into three
    logical buckets:

    - Unique features not present in the tracing infracture, in the
    event tracer or other tracing plugins - those should be
    structured as feature additions.

    - Features that you consider superior to existing tracing
    features of the kernel. For those, please iterate the
    existing code with your enhancements - instead of a parallel
    implementation.

    - Items which offer nothing new and are not superior to
    existing features, those should be dropped probably. This too
    is a case by case thing.

    Would you be interested in working with us on that? I know that
    both Steve and me would be very much interested in that. If you
    have time/interest to work on that then we can go through each
    patch one by one and categorize them and map out the way to go.

    Let me give you a few examples of existing areas of overlap:

    > The corresponding git tree contains also the trace clock
    > patches and the lttng instrumentation. The trace clock is
    > required to use the tracer, but it can be used without the
    > instrumentation : there is already a kprobes and userspace
    > event support included in this patchset.

    The latest tracing tree includes kernel/tracing/trace_clock.c
    which offers three trace clock variants, with different
    performance/precision tradeoffs:

    trace_clock_local() [ for pure CPU-local tracers with no idle
    events. This is the fastest but least
    coherent tracing clock. ]

    trace_clock() [ intermediate, scalable clock with
    usable but imprecise global coherency. ]

    trace_clock_global() [ globally serialized, coherent clock.
    It is the slowest but most accurate variant. ]

    Tracing plugins can pick their choice. (This is relatively new
    code but you get the idea.)

    > This tracer exports binary data through buffers using
    > splice(). The resulting binary files can be parsed from
    > userspace because the format string metadata is exported in
    > the files. The event set can be enhanced by adding tracepoints
    > to the kernel code and by creating probe modules, which
    > connects callbacks to the tracepoints and contain the format
    > string metainformation. Those callbacks are responsible for
    > writing the data in the trace buffers. This separation between
    > the trace buffer format string and the tracepoints is done on
    > purpose so the core kernel instrumentation (tracepoints) is
    > not exported to userspace, which will make maintainance much
    > easier.

    A tracepoint format specification mechanism plus working (and
    fast!) zero-copy splice() support of the ring-buffer exists in
    the latest tracing tree already - as you are probably aware of
    because you commented on those patches a few days ago.

    There are 3 good ways to go from here regarding the trace
    buffering and splice code:

    1- we end up switching to the lttng version in essence
    2- we end up keeping the tracing tree version
    3- we end up somewhere inbetween

    Which point in the above spectrum we will settle down on depends
    on the technical details.

    Note, whichever path we choose a gradual, iterative workflow is
    still needed, so that we improve the existing upstream code with
    lttng enhancements gradually.

    This approach works for all your other patches as well. A
    direct, constructive comparison and active work on unifying them
    is required.

    Thanks,

    Ingo


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