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

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Message-ID: <4CEFE1CB.4050404@kernel.org>
Date:	Fri, 26 Nov 2010 17:35:23 +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 11/24/2010 12:51 AM, Christoph Lameter wrote:
> this_cpu_inc_return() saves us a memory access there.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> ---
>  arch/x86/kernel/apic/nmi.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-2.6/arch/x86/kernel/apic/nmi.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/nmi.c	2010-11-23 16:35:19.000000000 -0600
> +++ linux-2.6/arch/x86/kernel/apic/nmi.c	2010-11-23 16:38:29.000000000 -0600
> @@ -432,8 +432,7 @@ nmi_watchdog_tick(struct pt_regs *regs,
>  		 * Ayiee, looks like this CPU is stuck ...
>  		 * wait a few IRQs (5 seconds) before doing the oops ...
>  		 */
> -		__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?

Thanks.

-- 
tejun
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