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    SubjectRe: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if they exist
    On Thu, Jan 05, 2012 at 04:17:39PM +0000, Mel Gorman wrote:
    > On Thu, Jan 05, 2012 at 02:40:11PM +0000, Russell King - ARM Linux wrote:
    > > On Thu, Jan 05, 2012 at 02:20:17PM +0000, Mel Gorman wrote:

    [ . . . ]

    > > I've been chasing that patch and getting no replies what so
    > > ever from folk like Peter, Thomas and Ingo.
    > >
    > > The problem affects all IPI-raising functions, which mask with
    > > cpu_online_mask directly.
    > Actually, in one sense I'm glad to hear it because from my brief
    > poking around, I was having trouble understanding why we were always
    > safe from sending IPIs to CPUs in the process of being offlined.

    The trick is to disable preemption (not interrupts!) across the IPI, which
    prevents CPU-hotplug's stop_machine() from running. You also have to
    have checked that the CPU is online within this same preemption-disabled
    section of code. This means that the outgoing CPU has to accept IPIs
    even after its CPU_DOWN_PREPARE notifier has been called -- right up
    to the stop_machine() call to take_cpu_down().

    Thanx, Paul

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