linux-kernel - Re: What is the difference between percpu_xxx and this_cpu

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Message-ID: <20110913074004.GA11397@mtj.dyndns.org>
Date:	Tue, 13 Sep 2011 16:40:04 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: What is the difference between percpu_xxx and this_cpu_xxx?

On Tue, Sep 13, 2011 at 02:14:06PM +0800, Huang Ying wrote:
> Hi, Eric,
> 
> On 09/13/2011 02:11 PM, Eric Dumazet wrote:
> > Le mardi 13 septembre 2011 à 12:28 +0800, Huang Ying a écrit :
> >> Hi,
> >>
> >> If my understanding was correct, from the kernel source code, it appears
> >> that percpu_xxx and this_cpu_xxx have the same semantics and
> >> implementation in effect, haven’t them?
> >>
> >> If so, why do we need two API for the same thing?
> > 
> > History mostly, and transition.
> > 
> > percpu_inc() was an x86 only thing
> > 
> > __get_cpu_var(var)++ -> percpu_inc(var) -> this_cpu_inc(var)

Also, we used to need separate accessors for static and dynamic percpu
variables, so we have quite a few duplications in the interface.

> Thanks for clarification.  Is there someone working on the transition?
> Is that possible to translate most percpu_xxx to this_cpu_xxx blindly?

It's on my todo list but AFAIK nobody is actually working on it yet.

Thanks.

-- 
tejun
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