netdev - Re: Poll about irqsafe_cpu

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Date:	Thu, 17 Mar 2011 19:29:29 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Poll about irqsafe_cpu_add and others

Le jeudi 17 mars 2011 à 12:46 -0500, Christoph Lameter a écrit :
> On Thu, 17 Mar 2011, Eric Dumazet wrote:
> 
> > When we know we run from BH context, we can use __this_cpu_inc(), but if
> > we dont know or run from user/process context, we would need irqsafe_inc
> > variant.
> 
> If the BH context is the only one where we care about performance then its
> ok I would think.

Hmm... yes.

By the way, I noticed :

DECLARE_PER_CPU(u64, xt_u64);
__this_cpu_add(xt_u64, 2) translates to following x86_32 code :

	mov	$xt_u64,%eax
	add	%fs:0x0,%eax
	addl	$0x2,(%eax)
	adcl	$0x0,0x4(%eax)


I wonder why we dont use :

	addl	$0x2,%fs:xt_u64
	addcl	$0x0,%fs:xt_u64+4



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