Date: Fri, 16 Jan 2009 01:15:44 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: akpm@linux-foundation.org, tj@kernel.org, hpa@zytor.com, brgerst@gmail.com,
       ebiederm@xmission.com, cl@linux-foundation.org, rusty@rustcorp.com.au,
       travis@sgi.com, linux-kernel@vger.kernel.org, steiner@sgi.com,
       hugh@veritas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
Message-ID: <20090116001544.GA11073@elte.hu>
References: <20090115183942.GA6325@elte.hu> <20090116001200.GA9137@gondor.apana.org.au>
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* Herbert Xu <herbert@gondor.apana.org.au> wrote:

> Ingo Molnar <mingo@elte.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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