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

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Message-ID: <4846B50F.5060808@sgi.com>
Date:	Wed, 04 Jun 2008 08:30:23 -0700
From:	Mike Travis <travis@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Christoph Lameter <clameter@....com>,
	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>
Subject: Re: [patch 02/41] cpu alloc: The allocator

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
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