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    SubjectRe: PROBLEM: Off-by-one bug on sun4u SMP
    From: Ryan Davis <>
    Date: Thu, 10 Jun 2010 11:05:09 -0700

    > On at least some sun4u machines, the CPU numbering starts at one not 0.
    > This causes an off-by-one bug as other parts of the code assume
    > zero-based numbering.
    > If you set max-cpus in the kernel config to the actual number of CPUs,
    > the last CPU will be ignored and unused.
    > This is because the CPU numbering starts at 1 but the code to check
    > against max-cpus assumes a zero-based numbering.
    > On my computer: Sun Ultra 60 2x450mhz Ultrasparc.
    > Building with max-cpus of 2 ignores the second cpu because 2 (one
    > based cpu number) >= 2 (zero based max cpus)
    > Rebuilding with a larger max-cpus is a workaround but non optimal.

    max-cpus means "one larger than the maximum PHYSICAL cpu number", not
    the number of cpus. That's what this setting means, at least on

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