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    SubjectRe: [PATCH RFC] sched: add notifier for process migration
    On 10/14/2009 06:26 PM, Peter Zijlstra wrote:
    >> We already have one event notifier there - look at the
    >> perf_swcounter_event() callback. Why add a second one for essentially
    >> the same thing?
    >>
    >> We should only put a single callback there - a tracepoint defined via
    >> TRACE_EVENT() - and any secondary users can register a callback to the
    >> tracepoint itself.
    >>
    >> There's many similar places in the kernel - with notifier chains and
    >> also with a need to get tracepoints there. The fastest (and most
    >> consistent) solution is to add just a single event callback facility.
    >>
    > But that would basically mandate tracepoints to be always enabled, do we
    > want to go there?
    >
    > I don't think the overhead of tracepoints is understood well enough,
    > Jason you poked at that, do you have anything solid on that?
    >
    > Also, I can imagine the embedded people to not want that.
    >
    > I really like perf and tracepoints to not become co-dependent until
    > tracepoint become mandatory for all configurations.
    >

    It would be cleanest to have both pvclock and tracepoints select
    migration notifiers, defaulting to off. Similarly both perf and kvm
    should use preemption notifiers (they do the same thing - switch
    per-task values into and out of cpu registers).

    --
    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.



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