linux-kernel - Re: [PATCH v6 2/3] percpu: add PERCPU_ATOM

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Message-Id: <8FA31B91-D361-4F98-A2D3-EFC5D877EDB1@gmail.com>
Date:	Tue, 3 Nov 2015 23:11:16 +0900
From:	Jungseok Lee <jungseoklee85@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	catalin.marinas@....com, will.deacon@....com, tj@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	james.morse@....com, takahiro.akashi@...aro.org,
	mark.rutland@....com, barami97@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/3] percpu: add PERCPU_ATOM_SIZE for a generic percpu area setup

On Nov 3, 2015, at 1:10 AM, Christoph Lameter wrote:

Dear Christoph,

> On Sun, 1 Nov 2015, Jungseok Lee wrote:
> 
>> There is no room to adjust 'atom_size' now when a generic percpu area
>> is used. It would be redundant to write down an architecture-specific
>> setup_per_cpu_areas() in order to only change the 'atom_size'. Thus,
>> this patch adds a new definition, PERCPU_ATOM_SIZE, which is PAGE_SIZE
>> by default. The value could be updated if needed by architecture.
> 
> What is atom_size? Why would you want a difference allocation size here?
> The percpu area is virtually mapped regardless. So you will have
> contiguous addresses even without atom_size.

I think Catalin have already written down a perfect explanation. I'd like
memory with an alignment greater than PAGE_SIZE. But, __per_cpu_offset[]
is PAGE_SIZE aligned under a generic setup_per_cpu_areas(). That is,
secondary cores cannot get that kind of space.

Thanks for taking a look at this doubtable change!

Best Regards
Jungseok Lee
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