linux-kernel - Re: [PATCH RFC] memory-hotplug: add automatic onlining policy for the newly added memory

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Date:	Wed, 16 Dec 2015 18:29:43 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
CC:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, Jonathan Corbet <corbet@....net>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Daniel Kiper <daniel.kiper@...cle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"David Rientjes" <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Gu Zheng <guz.fnst@...fujitsu.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	yanxiaofeng <yanxiaofeng@...pur.com>,
	Changsheng Liu <liuchangsheng@...pur.com>,
	Kay Sievers <kay@...y.org>
Subject: Re: [PATCH RFC] memory-hotplug: add automatic onlining policy for
 the newly added memory

On 2015/12/16 17:17, Vitaly Kuznetsov wrote:

> Xishi Qiu <qiuxishi@...wei.com> writes:
> 
>> On 2015/12/16 2:05, Vitaly Kuznetsov wrote:
>>
>>> Currently, all newly added memory blocks remain in 'offline' state unless
>>> someone onlines them, some linux distributions carry special udev rules
>>> like:
>>>
>>> SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", ATTR{state}="online"
>>>
>>> to make this happen automatically. This is not a great solution for virtual
>>> machines where memory hotplug is being used to address high memory pressure
>>> situations as such onlining is slow and a userspace process doing this
>>> (udev) has a chance of being killed by the OOM killer as it will probably
>>> require to allocate some memory.
>>>
>>> Introduce default policy for the newly added memory blocks in
>>> /sys/devices/system/memory/hotplug_autoonline file with two possible
>>> values: "offline" (the default) which preserves the current behavior and
>>> "online" which causes all newly added memory blocks to go online as
>>> soon as they're added.
>>>
>>> Cc: Jonathan Corbet <corbet@....net>
>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> Cc: Daniel Kiper <daniel.kiper@...cle.com>
>>> Cc: Dan Williams <dan.j.williams@...el.com>
>>> Cc: Tang Chen <tangchen@...fujitsu.com>
>>> Cc: David Vrabel <david.vrabel@...rix.com>
>>> Cc: David Rientjes <rientjes@...gle.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>>> Cc: Gu Zheng <guz.fnst@...fujitsu.com>
>>> Cc: Xishi Qiu <qiuxishi@...wei.com>
>>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>>> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>>> ---
>>> - I was able to find previous attempts to fix the issue, e.g.:
>>>   http://marc.info/?l=linux-kernel&m=137425951924598&w=2
>>>   http://marc.info/?l=linux-acpi&m=127186488905382
>>>   but I'm not completely sure why it didn't work out and the solution
>>>   I suggest is not 'smart enough', thus 'RFC'.
>>
>> + CC: 
>> yanxiaofeng@...pur.com
>> liuchangsheng@...pur.com
>>
>> Hi Vitaly,
>>
>> Why not use udev rule? I think it can online pages automatically.
>>
> 
> Two main reasons:
> 1) I remember someone saying "You never need a mouse in order to add
> another mouse to the kernel" -- but we  we need memory to add more
> memory. Udev has a chance of being killed by the OOM killer as
> performing an action will probably require to allocate some
> memory. Other than that udev actions are generally slow compared to what
> we can do in kernel.

Hi Vitaly,

So why we add memory when there is almost no free memory left?
I think the administrator should add memory when the free memory is low
or he should do something to stop free memory become worse.

> 
> 2) I agree with Kay that '... unconditional hotplug loop through
> userspace is absolutely pointless' (https://lkml.org/lkml/2013/7/25/354). 
> (... and I should had add him to CC, adding now). Udev maintainers
> refused to add a rule for unconditional memory onlining to udev and now
> linux distros have to carry such custom rules.
> 

If the administrator don't know how to config the udev, he could use sysfs
(echo 1 > /sys/devices/system/node/nodeXX/memoryXX/online) to online it,
or write a script to do this.

Thanks,
Xishi Qiu


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