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

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Message-ID: <4846AD84.2090800@sgi.com>
Date:	Wed, 04 Jun 2008 07:58:12 -0700
From:	Mike Travis <travis@....com>
To:	Christoph Lameter <clameter@....com>
CC:	Eric Dumazet <dada1@...mosbay.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

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
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