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    SubjectRe: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

    * Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

    > Ar Gwe, 2006-09-15 am 13:08 -0400, ysgrifennodd Frank Ch. Eigler:
    > > Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
    > > - where 1000-cycle int3-dispatching overheads too high
    >
    > Why are your despatching overheads 1000 cycles ? (and if its due to
    > int3 why are you using int 3 8))

    this is being worked on actively: there's the "djprobes" patchset, which
    includes a simplified disassembler to analyze common target code and can
    thus insert much faster, call-a-trampoline-function based tracepoints
    that are just as fast as (or faster than) compile-time, static
    tracepoints.

    there's no fundamental reason why INT3 should be the primary model of
    inserting kprobes. Sometimes we are unlucky and the code which we target
    is too complex - then we take a few hundred cycles of a penalty. If that
    piece of code is a really common destination then we can add a static
    marker in the source which both prepares parameters and inserts a
    sufficiently sized NOP (or a function call) to prepare things for fast
    dynamic tracing - but it should only be an optional performance helper
    that we have the freedom to zap.

    (kprobes can be thought of as a special "JIT", and there's no
    fundamental reason why it couldnt do almost arbitrary transformations on
    kernel code.)

    and there's alot more that kprobes/systemtap can do: it can be a method
    of extending the kernel along a 'plugin' model - without having to
    impact the kernel source! That way people can experiment with kernel
    extensions on live kernels, without the barrier of recompile/reboot.

    Ingo
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