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    SubjectRe: [RFC PATCH 1/3] Unified trace buffer

    * Ingo Molnar <mingo@elte.hu> wrote:

    > ftrace has the same robustness design as lockdep has: as little
    > external infrastructure dependencies as possible. And lockdep has
    > recursion checks too, and excessive amounts of paranoia all around the
    > place.
    >
    > Ftrace has the same robustness philosophy too, and yes, despite that
    > we judged cpu_clock() to be worth the risk, because accurate and fast
    > timestamps are a feature and we didnt want to duplicate.

    and note that there's another pragmatic argument: often we notice
    cpu_clock() bugs by looking at traces. I.e. people fixing trace
    timestamps _fix the scheduler_. Sometimes it is very hard to notice
    scheduling artifacts that happen due to small inaccuracies in
    cpu_clock().

    so there's continuous coupling between precise scheduling and good trace
    timestamps. I'd be willing to pay a lot more for that than the few
    (rather obvious...) robustness problems we had with sched_clock() in the
    past.

    anyway ... i'm not _that_ attached to the idea, we can certainly go back
    to the original ftrace method of saving raw TSC timestamps and
    postprocessing. I think users will quickly force us back to a more
    dependable clock, and if not then you were right and i was wrong ;-)

    In fact even when we used sched_clock() there were some artifacts: as
    you pointed it out we dont want to do per event cross-CPU
    synchronization by default as that is very expensive. Some people wanted
    GTOD clock for tracing and we very briefly tried that - but that was an
    utter maintenance nightmare in practice.

    Ingo


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