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

[Xishi Qiu]

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

> Xishi Qiu <qiuxishi@xxxxxxxxxx> 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@xxxxxxx>
>>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>> Cc: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
>>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>>> Cc: Tang Chen <tangchen@xxxxxxxxxxxxxx>
>>> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
>>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>>> Cc: Gu Zheng <guz.fnst@xxxxxxxxxxxxxx>
>>> Cc: Xishi Qiu <qiuxishi@xxxxxxxxxx>
>>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>>> Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>>> ---
>>> - 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@xxxxxxxxxx
>> liuchangsheng@xxxxxxxxxx
>>
>> 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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