some questions about kernel source

[John Mahoney]

On Fri, Feb 18, 2011 at 9:28 AM, loody <miloody at gmail.com> wrote:
> hi :
>
>
> 2011/2/18 John Mahoney <jmahoney at waav.com>:
>> On Thu, Feb 17, 2011 at 9:17 AM, loody <miloody at gmail.com> wrote:
>>> hi :-)
>>>
>>> 2011/2/16 Mulyadi Santosa <mulyadi.santosa at gmail.com>:
>>>> Hi :)
>>>>
>>>> On Wed, Feb 16, 2011 at 12:59, Rajat Jain <rajatjain at juniper.net> wrote:
>>>>> Hello loody,
>>>>>
>>>>>> 1. in kernel/trace, I always see "__read_mostly" at the end of
>>>>>> parameter is that a compiler optimization parameter?
>>>>>
>>>>> Yes, it is a hint to the compiler that the parameter is mostly read, thus if the compiler has to make a decision between optimizing one of the read / write paths, it will optimize the read path even at the expense of write path.
>>>>
>>>>
>>>> To be precise, they will be grouped into same cache line as much as
>>>> possible. By doing so, those cache line won't be invalidated so often
>>>> (keeping them "hot" :) hehehhe )
>>>
>>> I cannot find it on the gcc manual.
>>> is it a option in kernel for kernel usage?
>>> if so, where I can found them.
>>> If not, can I use it on normal user level program?
>>>
>>
>> It is a macro defined for x86 as:
>>
>> #define __read_mostly __attribute__((__section__(".data..read_mostly")))
>>
>> http://lxr.linux.no/linux+v2.6.37/arch/x86/include/asm/cache.h
> I found where you pointed out but I try to find out where is it in the
> mips arch.

It looks pretty new for mips and you may have older code.

$ git log -p cache.h
commit 1befdd5536e1500371f7f884d0f0ae528a519333
Author: David Daney <ddaney at caviumnetworks.com>
Date:   Thu Oct 14 12:36:49 2010 -0700

    MIPS: Implement __read_mostly
commit 1befdd5536e1500371f7f884d0f0ae528a519333
Author: David Daney <ddaney at caviumnetworks.com>
Date:   Thu Oct 14 12:36:49 2010 -0700

    MIPS: Implement __read_mostly

    Just do what everyone else is doing by placing __read_mostly things in
    the .data.read_mostly section.

    mips_io_port_base can not be read-only (const) and writable
    (__read_mostly) at the same time.  One of them has to go, so I chose
    to eliminate the __read_mostly.  It will still get stuck in a portion
    of memory that is not adjacent to things that are written, and thus
    not be on a dirty cache line, for whatever that is worth.

    Signed-off-by: David Daney <ddaney at caviumnetworks.com>
    To: linux-mips at linux-mips.org
    Patchwork: http://patchwork.linux-mips.org/patch/1702/
    Signed-off-by: Ralf Baechle <ralf at linux-mips.org>

As a side note I would not use this just because you think your
variable is read more than written. Premature optimization is a waste
of time, so make sure your on a hot path before even considering
something like this.


--
John