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

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Date:	Fri, 6 Jun 2008 09:48:24 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mike Travis <travis@....com>
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

On Thursday 05 June 2008 01:30:23 Mike Travis wrote:
> 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

Well, my thinking is that unless we do true dynamic per-cpu, this entire patch 
series is a non-starter :(

Once we have that, we can reopen this.  Then we'll discuss why we're writing a 
new allocator rather than using the existing one :)

Cheers,
Rusty.
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