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    SubjectRe: [PATCH] LTT for 2.5.38 1/9: Core infrastructure

    On Sun, 22 Sep 2002, Karim Yaghmour wrote:

    > > - remove the 'event registration' and callback stuff. It just introduces
    > > unnecessery runtime overhead. Use an include file as a registry of
    > > events instead. This will simplify things greatly.
    >
    > OK, basically then all the trace points call the trace driver directly.

    yes. And in fact i'd suggest to not make it a driver but create a new
    kernel/trace.c file - if it's a central mechanism then it should live in a
    central place.

    > > Why do you need a
    > > table of callbacks registered to an event? Nothing in your patches
    > > actually uses it ...
    >
    > True, nothing in the patches actually uses it as this point. This was
    > added with the mindset of letting other tools than LTT use the trace
    > points already provided by LTT.

    okay. The thing is that generic callbacks and data hooks in the task
    structure are an invitation for various types of abuses - security and GPL
    type abuses. People do get very nervous when seeing such stuff - eg. read
    back Christoph Hellwig's comment from a few weeks ago. It's a red flag for
    many people. Provide a clean and concentrated set of APIs, no callbacks,
    no unnecessery hooks. I can see the technical reasons why you have added
    it - it's in theory an extensible interface, but generally we tend to add
    such stuff when it's needed - if it's needed at all.

    > > Just use one tracing function that copies the
    > > arguments into a per-CPU ringbuffer. It's really just a few lines.
    >
    > Sure, the writing of data itself is trivial. The reason you find the
    > driver to be rather full is because of its need to do a couple of
    > extra operations:
    > - Get timestamp and use delta since begining of buffer to reduce
    > trace size. (i.e. because of the rate at which traces are filled, it's
    > essential to be able to cut down in the data written as much as possible).

    yes - but even this one can also be solved by providing 2-3 macros that
    each are hardcoded for one specific event length each - this should cover
    about 90% of the events. Plus perhaps a more generic entry to handle the
    longer/rarer event lengths, and the variable event length stuff.

    > - Filter events according to event mask.

    yes - this is handled by the event_allowed() function.

    > - Copy extra data in case of some events (e.g. filenames). (We're
    > working on ways to simplify this).

    are you sure you want to copy filenames? File descriptor and inode numbers
    ought to be enough.

    > - Synchronize with trace daemon to save trace data. (A single per-CPU
    > circular buffer may be useful when doing kernel devleopment, but user
    > tracing often requires N buffers).
    >
    > In addition, because this data is available from user-space, you need to
    > be able to deal with many buffers. For example, you don't want some
    > random user to know everything that's happening on the entire system for
    > obvious security reasons. So the tracer will need to be able to have
    > per-user and per-process buffers.

    in fact i have the feeling that you should not expose any of this to
    ordinary users. Performance measurements are to be done by administrator
    types - all this stuff has heavy memory allocation impact anyway.

    in exactly which cases do you want to have multiple trace buffers? A
    single (large enough if needed) buffer should be enough. This i think is
    one of the core issues of your design.

    Ingo

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