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    SubjectRe: [RFC] Fast assurate clock readable from user space and NMI handler
    * Daniel Walker (dwalker@mvista.com) wrote:
    > On Tue, 2007-02-27 at 02:38 -0500, Mathieu Desnoyers wrote:
    >
    > >
    > > I am concerned about the automatic fallback to the PIT when no other
    > > clock source is available. A clocksource read would be atomic when TSC
    > > or HPET are available, but would fall back on PIT otherwise. There
    > > should be some way to specify that a caller is only interested in atomic
    > > clock sources (if none are available, the call should simply return an
    > > error, or 0).
    > >
    > I'm not sure what you mean by using the RCU

    The original proposal of this thread uses a RCU (read-copy-update) style
    update of the previous 64 bits counter : it swaps a pointer (atomically)
    upon update by incrementing a word-sized counter that is used, by the
    reader, to get the offest in the array (with a modulo operation) for the
    current readable data and as a way to detect incorrect reads of
    overwritten information (we re-read the word-sized counter after having
    read the data structure to make sure is has not been incremented. If we
    detect an increment, we redo the whole operation).

    > > I still think that an RCU style update mechanism would be a good way to
    > > fix the current clocksource read issue. Another, slower and non NMI
    > > safe way to do this would be with a read seqlock and with IRQ disabling.
    >
    > , but the pit clocksource
    > does disable interrupts with a spin_lock_irqsave().
    >

    When I say "clocksource read issue", I am talking about
    race between the function you proposed earlier, which you say is used in
    -rt kernels for latency tracing (get_monotonic_cycles), and HPET and TSC
    "last cycles" updates.

    Mathieu

    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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