On Tue, Jul 17, 2012 at 12:39 AM, Mulyadi Santosa
[off-list ref] wrote:

Hi..

On Mon, Jul 16, 2012 at 6:22 PM, Aft nix [off-list ref] wrote:

quoted

Hi,

The macro expansion of __read_mostly :

#define __read_mostly __attribute__((__section__(".data..read_mostly"))

This one is from cache.h

__init:
#define __init          __section(.init.text) __cold notrace

from init.h

__exit:

#define __exit          __section(.exit.text) __exitused __cold notrace

After searching through net i have not found any good explanation of
what is happening there.

like others had said, it means, the variables marked as read mostly
are grouped into special section in kernel memory layout, named
read_mostly. It's a subsection inside "data", which is I believe
belong to initialized variable.

By grouping this into single special region, it is hoped that you
reduce (if possible none happen) cache invalidation. Such thing happen
when bits in a cache line are updated. In such occasion, the whole
cache line must be updated by re-fetching the related memory address.

Thus, if all these bits are read only, once fetched, it will stays in
cache line as long as possible.

PS: there is no guarantee that this grouping will effectively
works...that's why you need to align it to the size of cache line.

Hi Santosa,

This special section, "data..read_mostly" , is it maintained by
kernel? then Linker has to know this information. How that is done?
I mean lets say a.c is compiled into a.o. a.o already has the offsets
for its data. Linker resolves the final address. So how the linker is
invoked
in a way so that this data flagged with __read__mostly will end up in
the special section?

Thing is, i'm getting the idea what these macros mean, but not
understanding the mechanisms behind them.

cheers.

--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

-- 
-aft