linux-kernel - Re: [PATCH 0/5] Add movablecore

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Date:	Tue, 20 Nov 2012 19:25:13 +0800
From:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Tang Chen <tangchen@...fujitsu.com>, wency@...fujitsu.com,
	linfeng@...fujitsu.com, rob@...dley.net, laijs@...fujitsu.com,
	jiang.liu@...wei.com, kosaki.motohiro@...fujitsu.com,
	minchan.kim@...il.com, mgorman@...e.de, rientjes@...gle.com,
	yinghai@...nel.org, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 0/5] Add movablecore_map boot option.

On 11/20/2012 07:07 PM, Yasuaki Ishimatsu wrote:
> 2012/11/20 5:53, Andrew Morton wrote:
>> On Mon, 19 Nov 2012 22:27:21 +0800
>> Tang Chen <tangchen@...fujitsu.com> wrote:
>>
>>> This patchset provide a boot option for user to specify ZONE_MOVABLE 
>>> memory
>>> map for each node in the system.
>>>
>>> movablecore_map=nn[KMG]@ss[KMG]
>>>
>>> This option make sure memory range from ss to ss+nn is movable memory.
>>> 1) If the range is involved in a single node, then from ss to the 
>>> end of
>>>     the node will be ZONE_MOVABLE.
>>> 2) If the range covers two or more nodes, then from ss to the end of
>>>     the node will be ZONE_MOVABLE, and all the other nodes will only
>>>     have ZONE_MOVABLE.
>>> 3) If no range is in the node, then the node will have no ZONE_MOVABLE
>>>     unless kernelcore or movablecore is specified.
>>> 4) This option could be specified at most MAX_NUMNODES times.
>>> 5) If kernelcore or movablecore is also specified, movablecore_map 
>>> will have
>>>     higher priority to be satisfied.
>>> 6) This option has no conflict with memmap option.
>>
>> This doesn't describe the problem which the patchset solves.  I can
>> kinda see where it's coming from, but it would be nice to have it all
>> spelled out, please.
>>
>
>> - What is wrong with the kernel as it stands?
>
> If we hot remove a memroy, the memory cannot have kernel memory,
> because Linux cannot migrate kernel memory currently. Therefore,
> we have to guarantee that the hot removed memory has only movable
> memoroy.
>
> Linux has two boot options, kernelcore= and movablecore=, for
> creating movable memory. These boot options can specify the amount
> of memory use as kernel or movable memory. Using them, we can
> create ZONE_MOVABLE which has only movable memory.
>
> But it does not fulfill a requirement of memory hot remove, because
> even if we specify the boot options, movable memory is distributed
> in each node evenly. So when we want to hot remove memory which
> memory range is 0x80000000-0c0000000, we have no way to specify
> the memory as movable memory.

Could you explain why can't specify the memory as movable memory in this 
case?

>
> So we proposed a new feature which specifies memory range to use as
> movable memory.
>
>> - What are the possible ways of solving this?
>
> I thought 2 ways to specify movable memory.
>  1. use firmware information
>  2. use boot option
>
> 1. use firmware information
>   According to ACPI spec 5.0, SRAT table has memory affinity structure
>   and the structure has Hot Pluggable Filed. See "5.2.16.2 Memory
>   Affinity Structure". If we use the information, we might be able to
>   specify movable memory by firmware. For example, if Hot Pluggable
>   Filed is enabled, Linux sets the memory as movable memory.
>
> 2. use boot option
>   This is our proposal. New boot option can specify memory range to use
>   as movable memory.
>
>> - Describe the chosen way, explain why it is superior to alternatives
>
> We chose second way, because if we use first way, users cannot change
> memory range to use as movable memory easily. We think if we create
> movable memory, performance regression may occur by NUMA. In this case,

Could you explain why regression occur in details?

> user can turn off the feature easily if we prepare the boot option.
> And if we prepare the boot optino, the user can select which memory
> to use as movable memory easily.
>
> Thanks,
> Yasuaki Ishimatsu
>
>>
>> The amount of manual system configuration in this proposal looks quite
>> high.  Adding kernel boot parameters really is a last resort. Why was
>> it unavoidable here?
>>
>
>
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