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

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Date:	Fri, 16 Jan 2009 11:12:00 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	akpm@...ux-foundation.org, tj@...nel.org, hpa@...or.com,
	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 <mingo@...e.hu> wrote:
>
>> 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.

One of the things I'd like to see happen with this work is for
us to have a cheap per-cpu atomic counter that we can use for
SNMP stats.

If we can make the inc/add variants into a single instruction,
then it won't need to disable preemption or interrupts.

So if you could design the API such that we have a variant of
add/inc that automatically disables/enables preemption then we
can optimise that away on x86.

Thanks,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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