Re: [Patch] statistics infrastructure - update 10

[Andrew Morton <akpm@osdl.org>]

On Wed, 12 Jul 2006 18:45:08 +0200
Martin Peschke <mp3@de.ibm.com> wrote:

> On Wed, 2006-07-12 at 09:10 -0700, Andrew Morton wrote:
> > On Wed, 12 Jul 2006 14:27:39 +0200
> > Martin Peschke <mp3@de.ibm.com> wrote:
> > 
> > > +#define statistic_ptr(stat, cpu) \
> > > +	((struct percpu_data*)((stat)->data))->ptrs[(cpu)]
> > 
> > This would be the only part of the kernel which uses percpu_data directly -
> > everything else uses the APIs (ie: per_cpu_ptr()).  How come?
> 
> The API, i.e. per_cpu_ptr(), doesn't allow to assign a value to any of
> the pointers in struct percpu_data. I need that capability because I
> make use of cpu hotplug notifications to fix per-cpu data at run time.

Fair enough, I guess.

> With regard to memory footprint this is much more efficient than using
> alloc_percpu().

How much storage are we talking about here?  I find it a bit hard to work
that out.

> Is it be preferable to add something like set_per_cpu_ptr() to the API?

hm.  Add a generic extension to a generic interface within a specific
subsystem versus doing it generically.  Hard call ;)


I'd suggest that you:

- Create a new __alloc_percpu_mask(size_t size, cpumask_t cpus)

- Make that function use your newly added

	percpu_data_populate(struct percpu_data *p, int cpu, size_t size, gfp_t gfp);

	(maybe put `size' into 'struct percpu_data'?)

- implement __alloc_percpu() as __alloc_percpu_mask(size, cpu_possible_map)

- hack around madly until it compiles on uniprocessor.