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

[Vitaly Kuznetsov]

Xishi Qiu <qiuxishi@xxxxxxxxxx> writes:

> 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.

I have virtual machines use-case in my mind where hypervisor adds new
memory on high memory pressure reports from the guest (e.g. Hyper-V
behaves like that). This is an automatic action.

>
>> 
>> 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.

Oh, no, I'm not taking about manual actions here. My suggestion doesn't
eliminate this possibility and it doesn't even change the default --
memory blocks stay in 'offline' state unless someone requests the
auto-online policy.

-- 
  Vitaly
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