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    SubjectRe: [PATCH] perfcounters: Make s/w counters in a group only count when group is on

    * Peter Zijlstra <peterz@infradead.org> wrote:

    > On Fri, 2009-03-13 at 13:44 +0100, Peter Zijlstra wrote:
    >
    > > The same for task migration, most migrations happen when
    > > they are in fact running, so there too we can account the
    > > migration either before we rip it off the src cpu, or after
    > > we place it on the dst cpu.
    >
    > Right, I got confused between being on the cpu and having
    > ->state = R.
    >
    > Migrations are the odd one out indeed, but I'd rather fudge a
    > little with the migration counter itself than have this weird
    > asymmetry.

    Agreed. There should really be no difference between software
    and hardware counters as far as the generic perfcounters code
    goes. It's a magic "metric" that gets read out somehow, and
    which generates events somehow.

    We can have various grades of hardware versus software counters:

    - 'pure hardware counters' where both the count and events come
    from some hw register

    - 'pure software counters' where both the count and events are
    generated by software

    - 'hybride counters' where for example the count might be from a
    hardware register, but the event is generated by a hrtimer
    (because the hardware is not capable of generating events).

    the is_software_counter() assymetry broke this generally relaxed
    model of counters.

    Ingo


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