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    SubjectRe: [tip:perf/core] perf: Ignore non-sampling overflows
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    On Wed, 2011-06-29 at 12:37 +0200, Robert Richter wrote:

    > I looked at the interrupt handlers. The events are always determined
    > from a per-cpu array:
    >
    > cpuc = &__get_cpu_var(cpu_hw_events);
    > ...
    > event = cpuc->events[idx];
    >
    > In case of interrupts the event should then always be a hw event (or
    > uninitialized). Even if the interrupt was triggered by a different
    > source, it would always be mapped to the same event and the check
    > is_sampling_event() would be meaningless.

    I'm probably not quite getting what you mean, but how is
    is_sampling_event() meaningless? the INT bit is enabled for _all_
    events, whether they were configured as a sampling event or not.

    Its just that for !sampling events we shouldn't attempt to generate any
    output.

    > There are other occurrences of perf_event_overflow() in
    > kernel/events/core.c for events of type PERF_TYPE_SOFTWARE. These
    > events are initialized with sample_period set and a check would always
    > be true too.

    I'm failing to see what you mean, where do we always set
    event->attr.sample_period for software events?

    > For both cases I stil don't see a reason for the check.

    You're going to have to spell things out for me, I'm really not getting
    your argument.

    > Anyway, would the following extentension of the check above ok?
    >
    > if (unlikely(!is_sampling_event(event) && !event->attr.sample_type))
    > ...
    >
    > With no bits set in attr.sample_type the sample would be empty and
    > nothing to report. Now, with this change, samples that have data to
    > report wouldn't be dropped anymore.

    Also, could you explain in what way data is dropped? Where do
    non-sampling events need to write sample data?


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