Hi Srivatsa,

Thanks for the response. I have used per-CPU vars and I know about how
to creating/using per-CPU vars: DECLARE_PER_CPU(type, name) for
creating per-cpu at compile time and use alloc_percpu(type) for
creating them dynamically.

I intended to ask how they are stored internally (.percpu section) and
its protection mechanism if it has any.

Thanks & Regards,
Rajasekhar


On Fri, Mar 30, 2012 at 12:24 PM, Srivatsa S. Bhat
[off-list ref] wrote:

On 03/30/2012 12:05 PM, Dave Hylands wrote:

quoted

Hi Rajasekhar,

On Thu, Mar 29, 2012 at 11:00 PM, Rajasekhar Pulluru
[off-list ref] wrote:

quoted

Hi,

I would like to know how per-cpu data are stored internally?
And how are they protected from other cores?

To put it in very simplistic terms, per-cpu data is nothing but having
NR_CPUS copies of the data, like an array, something like:

int data[NR_CPUS];

And accessing this per-cpu data will essentially boil down to finding
out the id of the processor you are running on, and indexing this array
using that, something like:

int val, cpu;

cpu = smp_processor_id();
val = data[cpu];

So you automatically read/write the copy that belongs to your processor.
That's it. However, this is an over-simplified view of per-cpu data,
but you get the general idea...

quoted

I believe that they're just kmalloc'd like other kernel data. At the
kernel level there is no protection, just like all the rest of the
memory accessible to the kernel.
http://lxr.linux.no/#linux+v3.3/include/asm-generic/percpu.h#L8
http://lxr.linux.no/#linux+v3.3/mm/percpu.c

When you declare a per-cpu variable, it goes into a special section,
and what you're really doing is figuring out the offset within a
per_cpu region of memory.

Regards,
Srivatsa S. Bhat