PER-CPU data

[srivatsa.bhat@linux.vnet.ibm.com (Srivatsa S. Bhat)]

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

> Hi Rajasekhar,
> 
> On Thu, Mar 29, 2012 at 11:00 PM, Rajasekhar Pulluru
> <pullururajasekhar@gmail.com> wrote:
>> 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...

> 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