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    SubjectRe: perf_counter: request for three more sample data options
    Peter Zijlstra writes:

    > What I was thinking of was re-using some of the cpu_clock()
    > infrastructure. That provides us with a jiffy based GTOD sample,
    > cpu_clock() then uses TSC and a few filters to compute a current
    > timestamp.
    > I was thinking about cutting back those filters and thus trusting the
    > TSC more -- which on x86 can do any random odd thing. So provided the
    > TSC is not doing funny the results will be ok-ish.
    > This does mean however, that its not possible to know when its gone bad.

    I would expect that perfmon would be just reading the TSC and
    recording that. If you can read the TSC and do some correction then
    we're ahead. :)

    > The question to Paul is, does the powerpc sched_clock() call work in NMI
    > -- or hard irq disable -- context?

    Yes - timekeeping is one area where us powerpc guys can be smug. :)
    We have a per-core, 64-bit timebase register which counts at a
    constant frequency and is synchronized across all cores. So
    sched_clock works in any context on powerpc - all it does is read the
    timebase and do some simple integer arithmetic on it.


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