linux-kernel - Re: [cpuops inc_return V1 8/9] random: Use this_cpu_inc

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Message-ID: <alpine.DEB.2.00.1012061228010.5099@router.home>
Date:	Mon, 6 Dec 2010 12:29:46 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Matt Mackall <mpm@...enic.com>
cc:	Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
	Richard Kennedy <richard@....demon.co.uk>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops inc_return V1 8/9] random: Use this_cpu_inc_return

On Mon, 6 Dec 2010, Matt Mackall wrote:

> On Mon, 2010-12-06 at 11:40 -0600, Christoph Lameter wrote:
> > plain text document attachment (cpuops_inc_return_random)
> > __this_cpu_inc can create a single instruction to do the same as
> > __get_cpu_var()++.
>
> > -	    (__get_cpu_var(trickle_count)++ & 0xfff))
> > +	    ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff))
>
> I see you've added a "- 1" to mimic the post-increment ++ semantic.
>
> The goal of this code to add only 1 sample in every 4096, but we don't
> really care which, so this semantic isn't actually important here.

-1 also removes the + 1 that the inc_return() semantics require because we
use xadd on x86. -1 will generate more compact code.

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