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    SubjectRe: [PATCH 13/15] perf_counter: provide generic callchain bits
    Peter Zijlstra wrote:
    > On Tue, 2009-03-31 at 20:12 +1100, Paul Mackerras wrote:
    >> Corey Ashford writes:
    >>
    >>> If this is the case, what is the exact meaning of PERF_COUNTER_SIMPLE
    >>> now? and PERF_COUNTER_GROUP? One simplification would be that reading
    >>> the fd of a group leader would always read up all of the counters in the
    >>> group (along with their enabled and running times if those bits are
    >>> set), and that reading a single counter's fd would yield only that
    >>> counter's values and times (if enabled). In effect, these two values,
    >>> PERF_COUNTER_GROUP and PERF_COUNTER_SIMPLE would no longer be necessary
    >>> at all. Other bits would be used to determine what's in the mmap'd samples.
    >> Now that events are only read through mmap, it's quite simple -
    >> hw_event.read_format controls what read() gives you, and
    >> hw_event.record_type controls what gets put into the pages that you
    >> get with mmap.
    >>
    >> Currently read_format and record_type don't use the same set of bit
    >> definitions. Maybe they should? I don't have a strong feeling about
    >> it, but that might be a nice simplification.
    >
    > Something like the below?
    >
    > That still has the record and read things separate, but as one unified
    > overflow output.
    >
    > I reduced record and read fields to 32bits, as the output type only has
    > 32bits.
    >
    > diff did a wonderful job making the patch unreadable :/
    >

    This looks great! I like the idea of two bit vectors to describe what
    you get when you read and the format of the mmap'd data. And I like the
    ability not to include the header on each record if the user space
    program knows the exact format of the sample records.

    Paul is right, too, that having a read_format bit to determine whether
    reading the leader gives all of the counters or just the leader is a
    nice addition. I can't think of when I'd use it, but it's a good
    capability.

    - Corey



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