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    SubjectRe: [GIT RFC] percpu: use dynamic percpu allocator as the default percpu allocator
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    From: Tejun Heo <tj@kernel.org>
    Date: Tue, 10 Mar 2009 16:53:46 +0900

    > This patchset converts all SMP-capable arches other than three
    > non-trivial ones - powerpc64, sparc64 and ia64 - to use dynamic percpu
    > allocator. The affected archs are
    ...
    > The only differences are 1. there can be more space between percpu
    > areas for each cpu to accomodate minimum allocation size and first
    > chunk dynamic reserve and 2. dynamic percpu variables use the same
    > address translation mechanism as static ones.
    >
    > #1 shouldn't cause any trouble and #2 shouldn't too because the
    > offsets for dynamic percpu variables are carried in full pointer-width
    > variables, so as long as the calculations don't make wrong assumptions
    > (should be masked by RELOC_HIDE), it should be okay, but if your arch
    > has addressing limitations (alpha, s390), please take a second look.

    The one thing sparc64 does is calculate the base of the per-cpu
    area using a base (contained in a fixed global register) a shift.

    From arch/sparc/include/asm/percpu_64.h

    register unsigned long __local_per_cpu_offset asm("g5");
    extern unsigned long __per_cpu_base;
    extern unsigned long __per_cpu_shift;
    #define __per_cpu_offset(__cpu) \
    (__per_cpu_base + ((unsigned long)(__cpu) << __per_cpu_shift))
    #define per_cpu_offset(x) (__per_cpu_offset(x))
    #define __my_cpu_offset __local_per_cpu_offset

    I hope this won't cause problems for what you're trying to accomplish.


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