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    SubjectRe: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets
    On Friday, February 10, 2012, Alan Stern wrote:
    > On Fri, 10 Feb 2012, Peter Zijlstra wrote:
    > > Now the whole problem here seems to be that suspend uses cpu-hotplug to
    > > reduce the machine to UP -- I've no clue why it does that but I can
    > > imagine its because the BIOS calls only work on CPU0 and/or the resume
    > > only wakes CPU0 so you have to bootstrap the SMP thing again..

    Yes, that's the main problem.

    > > Some suspend person wanna clarify? Rafael?
    > If I understand correctly, ACPI requires that only CPU0 be running when
    > the system is suspended (i.e., your guess is right). Of course, this
    > doesn't apply to non-ACPI systems, but it's easiest to do the same
    > thing everywhere.

    That's correct. Plus we have some code in the kernel (syscore_ops in
    general) that assumes to be executed on one CPU with interrupts off.
    Plus I'm not sure how the ACPI low-level suspend is going to behave if it's
    not executed on the boot CPU.

    > > Anyway, the whole suspend case is magic anyway since all tasks will have
    > > been frozen, so we could simply leave all of cpuset alone and ignore the
    > > hotplug notifier on CPU_TASKS_FROZEN callbacks, hmm?
    > I don't see why not. Presumably no CPUs will be added or removed while
    > the system is asleep.

    ACPI explicitly forbids that level of hardware reconfiguration in a sleep
    state (even in S4), AFAICS. Still, people may try to do that ...


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