linux-kernel - Re: [patch 02/41] cpu alloc: The allocator

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Date:	Wed, 04 Jun 2008 17:11:00 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Mike Travis <travis@....com>
CC:	Christoph Lameter <clameter@....com>, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [patch 02/41] cpu alloc: The allocator

Mike Travis a écrit :
> Christoph Lameter wrote:
>> On Fri, 30 May 2008, Eric Dumazet wrote:
>>
>>>> +static DEFINE_PER_CPU(UNIT_TYPE, area[UNITS]);
>>>>   
>>> area[] is not guaranteed to be aligned on anything but 4 bytes.
>>>
>>> If someone then needs to call cpu_alloc(8, GFP_KERNEL, 8), it might get an non
>>> aligned result.
>>>
>>> Either you should add an __attribute__((__aligned__(PAGE_SIZE))),
>>> or take into account the real address of area[] in cpu_alloc() to avoid waste
>>> of up to PAGE_SIZE bytes
>>> per cpu.
>> I think cacheline aligning should be sufficient. People should not 
>> allocate large page aligned objects here.
> 
> I'm a bit confused.  Why is DEFINE_PER_CPU_SHARED_ALIGNED() conditioned on
> ifdef MODULE?
> 
>         #ifdef MODULE
>         #define SHARED_ALIGNED_SECTION ".data.percpu"
>         #else
>         #define SHARED_ALIGNED_SECTION ".data.percpu.shared_aligned"
>         #endif
> 
>         #define DEFINE_PER_CPU_SHARED_ALIGNED(type, name)                       \
>                 __attribute__((__section__(SHARED_ALIGNED_SECTION)))            \
>                 PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name             \
>                 ____cacheline_aligned_in_smp
> 
> Thanks,
> Mike
> 
> 

Because we had crashes when loading oprofile module, when a previous version of oprofile
used to use DEFINE_PER_CPU_SHARED_ALIGNED variable

module loader only takes into account the special section ".data.percpu" and ignores ".data.percpu.shared_aligned"

I therefore submitted two patches :

1) commit 8b8b498836942c0c855333d357d121c0adeefbd9
oprofile: don't request cache line alignment for cpu_buffer

Alignment was previously requested because cpu_buffer was an [NR_CPUS]
array, to avoid cache line sharing between CPUS.

After commit 608dfddd845da5ab6accef70154c8910529699f7 (oprofile: change
cpu_buffer from array to per_cpu variable ), we dont need to force an
alignement anymore since cpu_buffer sits in per_cpu zone.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
Cc: Mike Travis <travis@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>


2) and commit 	44c81433e8b05dbc85985d939046f10f95901184
per_cpu: fix DEFINE_PER_CPU_SHARED_ALIGNED for modules

Current module loader lookups ".data.percpu" ELF section to perform
per_cpu relocation.  But DEFINE_PER_CPU_SHARED_ALIGNED() uses another
section (".data.percpu.shared_aligned"), currently only handled in
vmlinux.lds, not by module loader.

To correct this problem, instead of adding logic into module loader, or
using at build time a module.lds file for all arches to group
".data.percpu.shared_aligned" into ".data.percpu", just use ".data.percpu"
for modules.

Alignment requirements are correctly handled by ld and module loader.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
Cc: Rusty Russell <rusty@...tcorp.com.au>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>



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