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    SubjectRe: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus -v5
    On 02/14/2012 01:24 AM, Tony Luck wrote:

    > On Thu, Feb 2, 2012 at 12:03 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
    >> IIRC playing with 3 archs boot code seemed like a recipe for disaster.
    >> Feel free to try to fix this in -next though, and see what breaks...
    >
    > ia64 is what breaks ... well not actually broken ... but some very
    > weird delays that
    > show up in different places depending on whether this patch is present.
    >
    > First linux-next kernel to be blessed with this patch was
    > next-20120210. Booting it
    > I see:
    > [ 7.164233] Switching to clocksource itc
    > [ 146.077315] pnp: PnP ACPI init
    >
    > An ugly 138.913 second delay. Digging in the code showed that the bad bits
    > happened inside stop_machine()
    >
    > Reverting just this patch makes this big delay disappear:
    >
    > [ 32.780232] Switching to clocksource itc
    > [ 32.832100] pnp: PnP ACPI init
    >
    > but notice that it takes 25 extra seconds to get to this point in the
    > boot (and while
    > we expect to save some time by not re-computing num_online_cpus each time we
    > need it ... this looks to be a lot more than I'd expect!)
    >


    Oh no!! ia64 directly uses cpu_set() and cpu_clear() on cpu_online_map!!
    Grr.. It means num_online_cpus can be different from the actual number of
    online cpus because it doesn't go through the set_cpu_online() path.. I haven't
    yet pin-pointed the exact problem, but this definitely doesn't look good...

    Regards,
    Srivatsa S. Bhat



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