linux-kernel - Re: A problem with percpu variable cpu

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Date:	Wed, 22 Feb 2012 19:13:25 +0800
From:	Tao Jiang <jiangtao.jit@...il.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: A problem with percpu variable cpu_number

Hi:

Thank you all.

So in boot_cpu_init(), it will always set bit 0 to these masks.
If the boot cpu is the first processor, it's the right case.
And if the BP is not the first one, is it wrong?
But can it happen that the BP is not cpu0?
Thank you.


2012/2/22 Brian Gerst <brgerst@...il.com>:
> On Tue, Feb 21, 2012 at 9:29 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> On 02/21/2012 08:41 PM, Tao Jiang wrote:
>>>
>>> Hi Cong Wang:
>>>
>>> I read the file vmlinux.lds.S in arch/x86/kernel
>>> section .data..percpu is between .init.data and .init.end
>>> Is that means these percpu variables will be freed after init?
>>
>>
>> % grep -e __init_begin -e __init_end -e __per_cpu_start -e __per_cpu_end
>> /boot/System.map
>> 0000000000000000 D __per_cpu_start
>> 0000000000014bc0 D __per_cpu_end
>> ffffffff81cf3000 D __init_begin
>> ffffffff81dfc000 R __init_end
>> % objdump -d -j .data..percpu vmlinux | grep cpu_number
>> 000000000000dc38 <cpu_number>:
>
> The .data..percpu section is placed in the init section, but x86-64 is
> a special case as noted below.  The boot cpu is pointed to the init
> percpu section until setup_per_cpu_areas() is called, when it switches
> to the regular percpu area.  The init percpu data is then freed with
> all other init data.
>
> The reason the percpu symbols start at virtual address 0 on x86-64 is
> because of the requirement that gs_base must be a canonical address
> (it cannot be a simple offset like x86-32).  But, the data is still
> loaded in the init section in memory.  See
> arch/x86/kernel/vmlinux.lds.S for the explanation of how the linker
> changes the program headers to set the virtual address to zero but
> keeps the load address in the init section.
>
> To answer the original question. in the case of cpu_number, it is set
> to zero in the init section because it doesn't have an explicit
> initializer.  Therefore the boot cpu will always read zero for the
> cpu_number, even before setup_per_cpu_areas() is called.
>
>
> --
> Brian Gerst
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