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

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Date:	Fri, 16 Jan 2009 23:09:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, tj@...nel.org, hpa@...or.com,
	brgerst@...il.com, ebiederm@...ssion.com, cl@...ux-foundation.org,
	travis@....com, linux-kernel@...r.kernel.org, steiner@....com,
	hugh@...itas.com
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors


* Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Friday 16 January 2009 10:42:00 Herbert Xu 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.
> 
> Yep, already on it.  It's called local_t; that's what it was originally 
> designed for.
> 
> Unfortunately, to use it efficiently, we need large per-cpu areas.

This makes no sense to me at all. Care to explain?

	Ingo
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