linux-kernel - Re: [PATCH v5 3/9] net: xfrm: use __this_cpu

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Message-ID: <20121114084355.GK22290@secunet.com>
Date:	Wed, 14 Nov 2012 09:43:55 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Shan Wei <shanwei88@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	David Miller <davem@...emloft.net>,
	NetDev <netdev@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Kernel-Maillist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/9] net: xfrm: use __this_cpu_read per-cpu helper

On Tue, Nov 13, 2012 at 08:36:00PM +0800, Shan Wei wrote:
> Steffen Klassert said, at 2012/11/13 18:48:
> > 
> > Ok, so please add a commit message to describe your changes.
> > 
> > Thanks.
> > 
> 
> [PATCH v5] net: xfrm: use __this_cpu_read per-cpu helper
> 
> this_cpu_ptr/this_cpu_read is faster than per_cpu_ptr(p, smp_processor_id()) 
> and can reduce  memory accesses.
> The latter helper needs to find the offset for current cpu,
> and needs more assembler instructions which objdump shows in following. 
> 
> this_cpu_ptr relocates and address. this_cpu_read() relocates the address
> and performs the fetch. this_cpu_read() saves you more instructions
> since it can do the relocation and the fetch in one instruction.
> 
> per_cpu_ptr(p, smp_processor_id())：
>   1e:   65 8b 04 25 00 00 00 00         mov    %gs:0x0,%eax
>   26:   48 98                           cltq
>   28:   31 f6                           xor    %esi,%esi
>   2a:   48 c7 c7 00 00 00 00            mov    $0x0,%rdi
>   31:   48 8b 04 c5 00 00 00 00         mov    0x0(,%rax,8),%rax
>   39:   c7 44 10 04 14 00 00 00         movl   $0x14,0x4(%rax,%rdx,1)
> 
> this_cpu_ptr(p)
>   1e:   65 48 03 14 25 00 00 00 00      add    %gs:0x0,%rdx
>   27:   31 f6                           xor    %esi,%esi
>   29:   c7 42 04 14 00 00 00            movl   $0x14,0x4(%rdx)
>   30:   48 c7 c7 00 00 00 00            mov    $0x0,%rdi
> 
> Signed-off-by: Shan Wei <davidshan@...cent.com>

Applied to ipsec-next, thanks!
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