linux-kernel - Re: [PATCH] modpost.c: Add .text.unlikely to TEXT

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Message-ID: <87ppxbxwzg.fsf@rustcorp.com.au>
Date:	Wed, 01 May 2013 11:49:47 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tom Rini <trini@...com>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] modpost.c: Add .text.unlikely to TEXT_SECTIONS

Tom Rini <trini@...com> writes:
> On 04/28/2013 10:59 PM, Rusty Russell wrote:
>> Tom Rini <trini@...com> writes:
>> 
>>> Recent gcc's may place functions into the .text.unlikely section and we
>>> need to check this section as well for section mismatches now otherwise
>>> we may have false negatives for this test.
>> 
>> Hmm, I don't think it's all that recent, is it?  I can find it back to
>> gcc 4.0.4:
>> 
>> `-freorder-functions'
>>      Reorder functions in the object file in order to improve code
>>      locality.  This is implemented by using special subsections
>>      `.text.hot' for most frequently executed functions and
>>      `.text.unlikely' for unlikely executed functions.  Reordering is
>>      done by the linker so object file format must support named
>>      sections and linker must place them in a reasonable way.
>> 
>>      Also profile feedback must be available in to make this option
>>      effective.  See `-fprofile-arcs' for details.
>> 
>>      Enabled at levels `-O2', `-O3', `-Os'.
>> 
>> The comment is the same in in gcc 4.7.
>> 
>> So is your real issue that this section is generated with
>> -fprofile-arcs, or has something changed with gcc 4.8, or...?
>
> I've started seeing this with Linaro based 4.7 toolchains.  I can go
> back through their releases and see when it starts showing up there if
> it helps.  I didn't add .text.hot as I didn't have that section at all,
> fwiw.

Weird, did you turn on CONFIG_GCOV_KERNEL?  AFAICT you shouldn't see
this section without that.

Thanks,
Rusty.
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