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    SubjectRe: [2/3 PATCH] Kprobes: User space probes support- readpage hooks
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    Andrew, the need is probably better stated as one where system-wide probing
    or tracing is possible. There are times where one needs a global view. Of
    course one can use multiple tools to obtain such data e.g. probes in kernel
    space strace in user space and so on. The advantages of supporting user
    probes are as follows:

    1) A single tool providing the data capture in a consistent manner eases
    the problems of correlation of events across multiple tools (for kernel and
    user space)
    2) The dynamic aspect allows ad hoc probepoints to be inserted where no
    existing instrumentation is provided (emergency debug scenario for
    example).
    3) The user probe distinguishes itself from all other externally managed
    tracing mechanisms in that probepoints are globally applied - i.e. without
    reference to PID. Compare this with ptrace breakpoints (hence strace and
    gdb) where tracepoints and breakpoints are localized to a specified set of
    processes. user-probes achieves this by design without the side effects
    (privatization of pages) that ptrace has. Again this supports the global
    view.
    4) user-probes also supports the registering of the probepoints before an
    the probed code is loaded. The clearly has advantages for catching
    initialization problems.


    A real life example of where this capability would have been very useful is
    with a performance problem I am currently investigating. It involves a GPFS
    + SAMBA + TCPIP + RDAC and some user-space video serving application. We
    are looking are where the latencies are accumulating in the system for the
    specific user application. It's a very hard problem. Having multiple tools
    serve up a partial view and having to coordinate these view from both data
    analysis and data gathering perspectives is a real nightmare.

    - -
    Richard J Moore
    IBM Advanced Linux Response Team - Linux Technology Centre
    MOBEX: 264807; Mobile (+44) (0)7739-875237
    Office: (+44) (0)1962-817072



    Andrew Morton
    <akpm@osdl.org>
    To
    21/03/2006 prasanna@in.ibm.com
    02:12 cc
    ak@suse.de, davem@davemloft.net,
    suparna@in.ibm.com, Richard J
    Moore/UK/IBM@IBMGB,
    linux-kernel@vger.kernel.org
    bcc

    Subject
    Re: [2/3 PATCH] Kprobes: User space
    probes support- readpage hooks






    Prasanna S Panchamukhi <prasanna@in.ibm.com> wrote:
    >
    > The basic idea is to insert probes on user applications which may or
    > may not be in memory, at the time of probe insertion.

    umm yes, but what for?

    What does this entire feature *do*? Why does Linux need it?

    OK, so it allows kernel modules to set breakpoints (via debug traps) into
    user code. But why do we want to be able to do that? What are the
    use-cases?

    This may sound like boringly obvious stuff to you, but without a complete
    problem statement from the designers, how are we to evaluate their proposed
    solution?



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