Hi, I would like to know how per-cpu data are stored internally? And how are they protected from other cores? Thanks & Regards, Rajasekhar
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?
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. -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com
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
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 <srivatsa.bhat@linux.vnet.ibm.com> wrote:
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
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
participants (3)
-
Dave Hylands -
Rajasekhar Pulluru -
Srivatsa S. Bhat