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    SubjectRe: [PATCH] perf/intel: Remove Perfmon-v4 counter_freezing support
    On Tue, Nov 10, 2020 at 12:52:04PM -0800, Stephane Eranian wrote:

    > What is implemented is Freeze-on-Overflow, yet it is described as Freeze-on-PMI.
    > That, in itself, is a problem. I agree with you on that point.

    Exactly.

    > However, there are use cases for both modes.
    >
    > I can sample on event A and count on B, C and when A overflows, I want
    > to snapshot B, C.
    > For that I want B, C at the moment of the overflow, not at the moment
    > the PMI is delivered. Thus, youd
    > would want the Freeze-on-overflow behavior. You can collect in this
    > mode with the perf tool,
    > IIRC: perf record -e '{cycles,instructions,branches:S}' ....

    Right, but we never supported that. Also, in that case the group must
    then be fully exlusive so as not to mess with other groups. A better
    solution might be an extention to Adaptive PEBS.

    > The other usage model is that of the replay-debugger (rr) which you are alluding
    > to, which needs precise count of an event including during the skid
    > window. For that, you need
    > Freeze-on-PMI (delivered). Note that this tool likely only cares about
    > user level occurrences of events.

    Correct, RR only cares about user-only counting.

    > As for counter independence, I am not sure it holds in all cases. If
    > the events are setup for user+kernel

    This is true; however if it were an actual Freeze-on-PMI we could
    actually do u+k independence correctly too.


    Anyway, as it stands I think the whole counter_freezing thing is a
    trainwreck and it needs to go.

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