linux-kernel - Re: [PATCH] percpu: add optimized generic percpu accessors

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Message-ID: <496FB05F.4020101@kernel.org>
Date:	Fri, 16 Jan 2009 06:53:35 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
	ebiederm@...ssion.com, cl@...ux-foundation.org,
	rusty@...tcorp.com.au, travis@....com,
	linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors

Ingo Molnar wrote:
>>> and "this is preempt safe" semantics.
>> Of course.  But do any architectures actually _need_ that for a single 
>> read?
> 
> not for a read i guess - but for the other ops like add/and/or/xor.

FWIW, it prevents cross-cpu cacheline contamination.  :-)

>> Maybe.  And if so, they can interpose their arch-specific 
>> implementation.  But if the generic version is optimal for them, they 
>> wouldn't need to..
> 
> we cannot turn the generic ops into a single instruction so arch methods 
> are preferable no matter how thick or thin the generic version is. But i 
> agree that the optimization you suggest could be done.

I think the preemption protection is good to have there for, if
nothing else, documentation.

Thanks.

-- 
tejun
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