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Date: Wed, 04 Jun 2008 08:30:23 -0700
From: Mike Travis <travis@sgi.com>
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To: Rusty Russell <rusty@rustcorp.com.au>
CC: Christoph Lameter <clameter@sgi.com>, Eric Dumazet <dada1@cosmosbay.com>,
       akpm@linux-foundation.org, linux-arch@vger.kernel.org,
       linux-kernel@vger.kernel.org, David Miller <davem@davemloft.net>,
       Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch 02/41] cpu alloc: The allocator
References: <20080530035620.587204923@sgi.com> <483F8AF7.9000309@cosmosbay.com> <Pine.LNX.4.64.0805292219220.12107@schroedinger.engr.sgi.com> <200805301552.25423.rusty@rustcorp.com.au>
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Rusty Russell wrote:
> On Friday 30 May 2008 15:20:45 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 vaguely recall there were issues with this in the module code.  They might 
> be gone now, but failing to meet alignment contraints without a big warning 
> would suck.
> 
> But modifying your code to consider the actual alignment is actually pretty 
> trivial, AFAICT.
> 
> Cheers,
> Rusty.

So paraphrasing my earlier email, we should add:

	bitmap_find_free_area(bitmap, nbits, size, align, alignbase)

so that > cacheline alignment is possible?

My thinking is that if we do go to true dynamically sized cpu_alloc area then
allocating PAGE_SIZE units may be both practical and worthwhile...?

Thanks,
Mike