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SubjectRe: [Patch 1/2] CPU hotplug compatible alloc_percpu
On Mon, 24 Jul 2006 19:16:54 +0200
Martin Peschke <mp3@de.ibm.com> wrote:

> This patch splits alloc_percpu() up into two phases. Likewise for
> free_percpu(). This allows clients to limit initial allocations to
> online cpu's, and to populate or depopulate per-cpu data at run time as
> needed:
>
> struct my_struct *obj;
>
> /* initial allocation for online cpu's */
> obj = percpu_alloc(sizeof(struct my_struct), GFP_KERNEL);
>
> ...
>
> /* populate per-cpu data for cpu coming online */
> ptr = percpu_populate(obj, sizeof(struct my_struct), GFP_KERNEL, cpu);
>
> ...
>
> /* access per-cpu object */
> ptr = percpu_ptr(obj, smp_processor_id());
>
> ...
>
> /* depopulate per-cpu data for cpu going offline */
> percpu_depopulate(obj, cpu);
>
> ...
>
> /* final removal */
> percpu_free(obj);

That looks pretty thorough.

The one little nit I'd have is that the code passes cpumasks by value. See
the tricks in <linux/cpumask.h> which pretend to take the caller's cpumask
by value but which instead pass it via const reference to the callee.

CONFIG_NR_CPUS=1024 leads to a 128-byte cpumask_t. It's worth doing.
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