Re: good explanation of __read_mostly, __init, __exit macros,

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On Tue, Jul 17, 2012 at 12:39 AM, Mulyadi Santosa
<mulyadi.santosa@xxxxxxxxx> wrote:
> Hi..
>
> On Mon, Jul 16, 2012 at 6:22 PM, Aft nix <aftnix@xxxxxxxxx> wrote:
>> 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

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