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

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Message-ID: <20090116001544.GA11073@elte.hu>
Date:	Fri, 16 Jan 2009 01:15:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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


* Herbert Xu <herbert@...dor.apana.org.au> wrote:

> 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.

Yeah. percpu_add(var, 1) does exactly that on x86.

	Ingo
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