linux-kernel - Re: [PATCH v2] memory-hotplug: add automatic onlining policy for the newly added memory

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Date:	Tue, 22 Dec 2015 13:55:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:	linux-mm@...ck.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.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>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Xishi Qiu <qiuxishi@...wei.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Kay Sievers <kay@...y.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2] memory-hotplug: add automatic onlining policy for
 the newly added memory

On Tue, 22 Dec 2015 17:32:30 +0100 Vitaly Kuznetsov <vkuznets@...hat.com> 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" 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 "online" when MEMORY_HOTPLUG_AUTOONLINE kernel config option
> is selected.

I think the default should be "offline" so vendors can ship kernels
which have CONFIG_MEMORY_HOTPLUG_AUTOONLINE=y while being
back-compatible with previous kernels.

> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2537,6 +2537,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			shutdown the other cpus.  Instead use the REBOOT_VECTOR
>  			irq.
>  
> +	nomemhp_autoonline	Don't automatically online newly added memory.
> +

This wasn't mentioned in the changelog.  Why do we need a boot
parameter as well as the sysfs knob?

> +config MEMORY_HOTPLUG_AUTOONLINE
> +	bool "Automatically online hot-added memory"
> +	depends on MEMORY_HOTPLUG_SPARSE
> +	help
> +	  When memory is hot-added, it is not at ready-to-use state, a special

"When memory is hot-added it is not in a ready-to-use state.  A special"

> +	  userspace action is required to online the newly added blocks. With
> +	  this option enabled, the kernel will try to online all newly added
> +	  memory automatically.
> +
>
> ...
>
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