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    SubjectRe: [PATCH 1/4 tip/sched/core] sched: rename preempt_notifier to sched_notifier and always enable it

    * Tejun Heo <tj@kernel.org> wrote:

    > Hello, Peter, Ingo.
    >
    > 11/26/2009 09:40 PM, Peter Zijlstra wrote:
    > > CALLBACK_EVENT() would be my preferred name, and shouldn't live anywhere
    > > near the regular tracing bits, the tracing bits could simply add another
    > > callback in it when enabled.
    >
    > I haven't looked at the mm code but if the scheduler callback
    > requirement isn't gonna explode big time soon and we know which
    > functions are the candidate callbacks at build time, I think this can
    > be done pretty efficiently with an ulong enable mask per task and
    > fixed function dispatch such that no callback case just goes through
    > one likely() conditional test at the tracing point and callback cases
    > are dispatched using conditional direct jump.

    Yes - and that's what the tracepoints infrastructure is about.

    Btw., longer term it will be faster than a mask check and a
    default-untaken conditional: there's ongoign work to offer runtime
    instruction patching features for tracing callbacks. There's the jump
    patching optimization and also the immediate values patching
    optimization.

    We've got old-style notifiers for regular callbacks, we've got new-style
    tracepoints which are callbacks and event source descriptors - and what
    i'm asking for is to have _one_ callback mechanism, and to use that in
    the scheduler. 5 callbacks using 3 different facilities is excessive -
    i'd like to see just two callbacks using one facility.

    Ingo


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