Date: Wed, 15 Jul 2015 18:13:45 -0400
From: Tejun Heo <tj@kernel.org>
To: Tang Chen <tangchen@cn.fujitsu.com>
Cc: mingo@redhat.com, akpm@linux-foundation.org, rjw@rjwysocki.net,
        hpa@zytor.com, laijs@cn.fujitsu.com, yasu.isimatu@gmail.com,
        isimatu.yasuaki@jp.fujitsu.com, kamezawa.hiroyu@jp.fujitsu.com,
        izumi.taku@jp.fujitsu.com, gongzhaogang@inspur.com,
        qiaonuohan@cn.fujitsu.com, x86@kernel.org, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 0/5] Make cpuid <-> nodeid mapping persistent.
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Hello,

On Tue, Jul 07, 2015 at 05:30:20PM +0800, Tang Chen wrote:
> [Solution]
> 
> To fix this problem, we establish cpuid <-> nodeid mapping for all the possible
> cpus at boot time, and make it invariable. And according to init_cpu_to_node(),
> cpuid <-> nodeid mapping is based on apicid <-> nodeid mapping and cpuid <-> apicid
> mapping. So the key point is obtaining all cpus' apicid.
> 
> apicid can be obtained by _MAT (Multiple APIC Table Entry) method or found in
> MADT (Multiple APIC Description Table). So we finish the job in the following steps:
> 
> 1. Enable apic registeration flow to handle both enabled and disabled cpus.
>    This is done by introducing an extra parameter to generic_processor_info to let the
>    caller control if disabled cpus are ignored.
> 
> 2. Introduce a new array storing all possible cpuid <-> apicid mapping. And also modify
>    the way cpuid is calculated. Establish all possible cpuid <-> apicid mapping when
>    registering local apic. Store the mapping in the array introduced above.
> 
> 4. Enable _MAT and MADT relative apis to return non-presnet or disabled cpus' apicid.
>    This is also done by introducing an extra parameter to these apis to let the caller
>    control if disabled cpus are ignored.
> 
> 5. Establish all possible cpuid <-> nodeid mapping.
>    This is done via an additional acpi namespace walk for processors.

Hmmm... given that we probably want to allocate lower ids to the
online cpus, as otherwise we can end up failing to bring existing cpus
online because NR_CPUS is lower than the number of possible cpus, I
wonder whether doing this lazily could be better / easier.  e.g. just
remember the mapping as cpus come online.  When a new cpu comes up,
look up whether it came up before.  If so, use the ids from the last
time.  If not, allocate new ones.  I think that would be less amount
of change but does require updating the mapping dynamically.

Thanks.

-- 
tejun
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