linux-kernel - Re: [cpuops cmpxchg V1 2/4] x86: this_cpu_cmpxchg and this_cpu

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Message-ID: <alpine.DEB.2.00.1012091739480.29367@router.home>
Date:	Thu, 9 Dec 2010 17:40:27 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [cpuops cmpxchg V1 2/4] x86: this_cpu_cmpxchg and this_cpu_xchg
 operations

On Wed, 8 Dec 2010, H. Peter Anvin wrote:

> On 12/08/2010 10:17 AM, Mathieu Desnoyers wrote:
> >
> > Hi Christoph,
> >
> > Can you show if this provides savings in terms of:
> >
> > - instruction cache footprint
> > - cycles required to run
> > - large-scale impact on the branch prediction buffers
> >
> > Given that this targets per-cpu data only, the additional impact on cache-line
> > exchange traffic of using cmpxchg over xchg (cache-line not grabbed as exclusive
> > by the initial read) should not really matter.
> >
> > I'm CCing Arjan and HPA, because they might have some interesting insight into
> > the performance impact of lock-prefixed xchg vs using local cmpxchg in a loop.
> >
>
> XCHG is always locked; it doesn't need the prefix.  Unfortunately,
> unlike on the 8086 on modern processors locks have a real cost.

So should we use xchg or a loop using prefixless cmpxchg instead when
referring to per cpu data and requiring only per cpu atomicness?

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