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

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Message-ID: <4CEFE8F6.5050109@kernel.org>
Date:	Fri, 26 Nov 2010 18:05:58 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
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 11/26/2010 06:02 PM, Christoph Lameter wrote:
> 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.

Thanks for the explanation.  It might be beneficial to note
performance characteristics on top of the x86 implementation?
Anyways, for this and the following simple conversion patches.

Reviewed-by: Tejun Heo <tj@...nel.org>

-- 
tejun
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