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

Mike Travis
2008-06-04T15:30:35Z

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