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    SubjectRe: [PATCH] SLUB use cmpxchg_local
    * Mathieu Desnoyers ( wrote:
    > * Christoph Lameter ( wrote:
    > > On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
    > >
    > > > - Changed smp_rmb() for barrier(). We are not interested in read order
    > > > across cpus, what we want is to be ordered wrt local interrupts only.
    > > > barrier() is much cheaper than a rmb().
    > >
    > > But this means a preempt disable is required. RT users do not want that.
    > > Without preemption the processor can be moved after c has been determined.
    > > That is why the smp_rmb() is there.
    > preemption is required if we want to use cmpxchg_local anyway.
    > We may have to find a way to use preemption while being able to give an
    > upper bound on the preempt disabled execution time. I think I got a way
    > to do this yesterday.. I'll dig in my patches.

    Yeah, I remember having done so : moving the preempt disable nearer to
    the cmpxchg, checking if the cpuid has changed between the
    raw_smp_processor_id() read and the preempt_disable done later, redo if
    it is different. It makes the slow path faster, but makes the fast path
    more complex, therefore I finally dropped the patch. And we talk about
    ~10 cycles for the slow path here, I doubt it's worth the complexity
    added to the fast path.

    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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