Re: [Patch 1/2] CPU hotplug compatible alloc_percpu

[Andrew Morton <akpm@osdl.org>]

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.