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    SubjectRe: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus -v5
    On 02/14/2012 01:55 AM, Srivatsa S. Bhat wrote:

    > On 02/14/2012 01:24 AM, Tony Luck wrote:
    >> On Thu, Feb 2, 2012 at 12:03 PM, Rusty Russell <> 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...

    Hmm.. interesting.. The only calls that ia64 uses which updates the
    num_online_cpus macro seem to be init_cpu_online(cpumask_of(0)); Atleast this
    is what the mainline code tells me (haven't checked linux-next).

    So, if I am not mistaken, is the value of num_online_cpus() always 1 when
    Venki's patch is applied?

    IOW, what output do you see from the following printk from

    printk(KERN_INFO "Total of %d processors activated (%lu.%02lu BogoMIPS).\n",
    (int)num_online_cpus(), bogosum/(500000/HZ), (bogosum/(5000/HZ))%100);

    Srivatsa S. Bhat

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