linux-kernel - Re: [PATCH 0/9 v2] use efficient this_cpu

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Date:	Fri, 2 Nov 2012 17:49:12 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Shan Wei <shanwei88@...il.com>
cc:	David Miller <davem@...emloft.net>,
	NetDev <netdev@...r.kernel.org>,
	Kernel-Maillist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/9 v2] use efficient this_cpu_* helper

On Sat, 3 Nov 2012, Shan Wei wrote:

> this_cpu_ptr 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.
>
> 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

this_cpu_read() etc even avoids the use of this_cpu_ptr()
reducing the 6 instructions earlier to 1.