linux-kernel - Re: [thiscpuops upgrade 05/10] x86: Use this_cpu_inc

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Message-ID: <alpine.DEB.2.00.1011261047460.13524@router.home>
Date:	Fri, 26 Nov 2010 11:02:45 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <tj@...nel.org>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [thiscpuops upgrade 05/10] x86: Use this_cpu_inc_return for nmi
 counter

On Fri, 26 Nov 2010, Tejun Heo wrote:

> > -		__this_cpu_inc(alert_counter);
> > -		if (__this_cpu_read(alert_counter) == 5 * nmi_hz)
> > +		if (__this_cpu_inc_return(alert_counter) == 5 * nmi_hz)
>
> Hmmm... one worry I have is that xadd, being not a very popular
> operation, might be slower than add and read.  Using it for atomicity
> would probably be beneficial in most cases but have you checked this
> actually is cheaper?

XADD takes 3 uops. INC 1 and MOV 1 uop. So there is an additiona uop.

However, a memory fetch from l1 takes a mininum 4 cycles. Doing that twice
already ends up with at least 8 cycles.




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