Re: [Xen-devel] [PATCH v4 1/2] memory-hotplug: add automatic onlining policy for the newly added memory

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Tue, Jan 12, 2016 at 05:56:16PM +0100, 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/auto_online_blocks file with two possible
> values: "offline" which preserves the current behavior and "online" which
> causes all newly added memory blocks to go online as soon as they're added.
> The default is "offline".
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  Documentation/memory-hotplug.txt | 19 +++++++++++++++----
>  drivers/base/memory.c            | 40 
> ++++++++++++++++++++++++++++++++++++----
>  drivers/xen/balloon.c            |  2 +-
>  include/linux/memory.h           |  3 ++-
>  include/linux/memory_hotplug.h   |  4 +++-
>  mm/memory_hotplug.c              | 18 +++++++++++++++---
>  6 files changed, 72 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/memory-hotplug.txt 
> b/Documentation/memory-hotplug.txt
> index ce2cfcf..ceaf40c 100644
> --- a/Documentation/memory-hotplug.txt
> +++ b/Documentation/memory-hotplug.txt
> @@ -254,12 +254,23 @@ If the memory block is online, you'll read "online".
>  If the memory block is offline, you'll read "offline".
>
>
> -5.2. How to online memory
> +5.2. Memory onlining
>  ------------
> -Even if the memory is hot-added, it is not at ready-to-use state.
> -For using newly added memory, you have to "online" the memory block.
> +When the memory is hot-added, the kernel decides whether or not to "online"
> +it according to the policy which can be read from "auto_online_blocks" file:
>
> -For onlining, you have to write "online" to the memory block's state file as:
> +% cat /sys/devices/system/memory/auto_online_blocks
> +
> +The default is "offline" which means the newly added memory is not in a
> +ready-to-use state and you have to "online" the newly added memory blocks
> +manually. Automatic onlining can be requested by writing "online" to
> +"auto_online_blocks" file:
> +
> +% echo online > /sys/devices/system/memory/auto_online_blocks

It looks that you forgot to mention that setting auto_online_blocks to
online does not online currently offlined blocks automatically.

Daniel

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