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    SubjectRe: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function
    On Sat, 14 Nov 2009, Wu Zhangjin wrote:

    > From: Wu Zhangjin <wuzhangjin@gmail.com>
    >
    > trace_clock_local() is based on the arch-specific sched_clock(), in X86,
    > it is tsc(64bit) based, which can give very high precision(about 1ns
    > with 1GHz). but in MIPS, the sched_clock() is jiffies based, which can
    > give only 10ms precison with 1000 HZ. which is not enough for tracing,
    > especially for Real Time system.
    >
    > so, we need to implement a MIPS specific sched_clock() to get higher
    > precision. There is a tsc like clock counter register in MIPS, whose
    > frequency is half of the processor, so, if the cpu frequency is 800MHz,
    > the time precision reaches 2.5ns, which is very good for tracing, even
    > for Real Time system.
    >
    > but 'Cause it is only 32bit long, which will rollover quickly, so, such
    > a sched_clock() will bring with extra load, which is not good for the
    > whole system. so, we only need to implement a arch-specific
    > trace_clock_local() for tracing. as a preparation, we convert it as a
    > weak function.

    Hmm, I'm not convinced that this is really a huge overhead.

    First of all the rollover happens once every 10 seconds on a 800MHz
    machine.

    Secondly we have a lockless implementation of extending 32bit counters
    to 63 bit which is used at least by ARM to provide a high resolution
    sched_clock implementation. See include/linux/cnt32_63.h and the users
    in arch/

    But that's a problem which can be discussed seperately and does not
    affect the rest of the tracing infrastructure. I really would
    recommend that you implement a sched_clock for the r4k machines based
    on cnt32_63 and measure the overhead. Having a fine granular
    sched_clock in general is probably not a bad thing.

    Thanks,

    tglx


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