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    SubjectRe: How to allocate per-cpu data for online CPUs only (and safely)?
    On Tue, Feb 21, 2006 at 11:29:02PM +0100, Martin Peschke wrote:
    > I am trying to optimize the memory footprint of some code of mine.
    > the disadvantage of getting hold of memory for CPU's which aren't there
    > (yet).
    > I could imagine using CPU-hotplug notifications as triggers for
    > additional allocations or for cleaning up unneeded memory. But
    > alloc_percpu() appears to conflict with that idea.
    > I was briefly tempted to derive some code from alloc_percpu() more to my
    > liking, until I was scared off by this comment in alloc_percpu():
    > /*
    > * Cannot use for_each_online_cpu since a cpu may come online
    > * and we have no way of figuring out how to fix the array
    > * that we have allocated then....
    > */
    > Well, and then there is kernel/profile.c, for example, which boldly
    > ignors alloc_percpu()'s qualms and allocates and releases per-cpu data
    > as needed.
    > Is that the way to go?
    > If so, why alloc_percpu()'s reservations?

    The above comment in alloc_percpu regarding use of for_each_online_cpu
    probably refers to the fact that once we handover 'pdata' to the caller
    of alloc_percpu, there is no way to go back and fix it (i.e allocate
    objects) for CPUs that come online later. This could have been possible if
    alloc_percpu kept track of all per-CPU allocations (chain togther all pdata's
    in a linked list?), but I guess they didn't do it to keep it simple.

    I would say the code in kernel/profile.c is fine enough to emulate.

    > Or, does that comment imply that the exploiter isn't expected to take
    > care of CPU hotplug events?
    > Am I missing anything?
    > Thank you.
    > Martin

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