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    SubjectRe: [PATCH] markers-linker-generic
    On Wed, 2007-04-11 at 15:21 -0400, Mathieu Desnoyers wrote:
    > * Andrew Morton ( wrote:
    > > On Wed, 11 Apr 2007 13:51:11 -0400
    > > Mathieu Desnoyers <> wrote:
    > >
    > > > > What's this marker stuff about?
    > > > >
    > > >
    > > > Hi Russel,
    > > >
    > > > Here is an overview :
    > >
    > > I am told that the systemtap developers plan to (or are) using this
    > > infrastructure.
    > >
    > Quoting Frank Ch. Eigler, from the SystemTAP team :
    > "The LTTng user-space programs use it today. Systemtap used to support
    > the earlier marker prototype and will be rapidly ported over to this
    > new API upon acceptance."
    > > If correct: what is their reason for preferring it over kprobes?
    > >
    > I will let them answer on this one..

    I'll take a shot at this one.

    First of all, kprobes remains a vital foundation for SystemTap. But
    markers are attactive as an alternate source of trace/debug info.
    Here's why:

    1. Markers will live in the kernel and presumably be kept up to date by
    the maintainers of the enclosing code. We have a growing set of tapsets
    (probe libraries), each of which "knows" the source code for a certain
    area of the kernel. Whenever the underlying kernel code changes (e.g.,
    a function or one of its args disappears or is renamed), there's a
    chance that the tapset will become invalid until we bring it back in
    sync with the kernel. As you can imagine, maintaining tapsets separate
    from the kernel source is a maintenance headache. Markers could
    mitigate this.

    2. Because the kernel code is highly optimized, the kernel's dwarf info
    doesn't always accurately reflect which variables have which values on
    which lines (sometimes even upon entry to a function). A marker is a
    way to ensure that values of interest are available to SystemTap at
    marked points.

    3. Sometimes the overhead of a kprobe probepoint is too much (either in
    terms of time or locking) for the particular hotspot we want to probe.


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