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    From
    SubjectRe: 2.6.16-rc6-mm2
    Date
    Andrew Morton (on Tue, 13 Jun 2006 04:45:32 -0700) wrote:
    >On Tue, 13 Jun 2006 15:08:40 +1000
    >Keith Owens <kaos@sgi.com> wrote:
    >
    >> Andi Kleen (on Tue, 13 Jun 2006 06:56:45 +0200) wrote:
    >> >
    >> >> I have previously suggested a lightweight solution that pins a process
    >> >> to a cpu
    >> >
    >> >That is preempt_disable()/preempt_enable() effectively
    >> >It's also light weight as much as these things can be.
    >>
    >> The difference being that preempt_disable() does not allow the code to
    >> sleep. There are some places where we want to use cpu local data and
    >> the code can tolerate preemption and even sleeping, as long as the
    >> process schedules back on the same cpu.
    >
    >It would be easy to use this mechanism wrongly:

    Agreed.

    > thread 1 on CPU N thread 2 on CPU N
    >
    > foo = per_cpu(...)
    > <preempt>
    > foo = per_cpu(...);
    > foo++;
    > per_cpu(...) = foo;
    > <unpreempt>
    > foo++;
    > per_cpu(...) = foo; // whoops
    >
    >
    >In which scenarios did you envisage it being used?

    There are not many scenarios where this makes any sense. One is where
    the code is working on a collection of related cpu data and the whole
    collection is protected by a per cpu mutex. Taking the mutex stops
    your race. I doubt if we have any code like that yet.

    The other possibility is to allow work to preempt the current process
    while it spins in udelay(). This is a problem on systems that use the
    cycle counter (TSC, ITC) and different cpus run at different rates.
    See http://marc.theaimsgroup.com/?l=linux-ia64&m=113460274218885&w=2

    I am not going to be too persistent about this facility. If it seems
    too risky or of too little use, then forget it.

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