Re: [Xen-devel] [PATCH RFC] mm/memory_hotplug: Introduce memory block types

[David Hildenbrand <david@xxxxxxxxxx>]

On 26.11.18 15:20, Michal Suchánek wrote:
> On Mon, 26 Nov 2018 14:33:29 +0100
> David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
>> On 26.11.18 13:30, David Hildenbrand wrote:
>>> On 23.11.18 19:06, Michal Suchánek wrote:  
> 
>>>>
>>>> If we are going to fake the driver information we may as well add the
>>>> type attribute and be done with it.
>>>>
>>>> I think the problem with the patch was more with the semantic than the
>>>> attribute itself.
>>>>
>>>> What is normal, paravirtualized, and standby memory?
>>>>
>>>> I can understand DIMM device, baloon device, or whatever mechanism for
>>>> adding memory you might have.
>>>>
>>>> I can understand "memory designated as standby by the cluster
>>>> administrator".
>>>>
>>>> However, DIMM vs baloon is orthogonal to standby and should not be
>>>> conflated into one property.
>>>>
>>>> paravirtualized means nothing at all in relationship to memory type and
>>>> the desired online policy to me.  
>>>
>>> Right, so with whatever we come up, it should allow to make a decision
>>> in user space about
>>> - if memory is to be onlined automatically  
>>
>> And I will think about if we really should model standby memory. Maybe
>> it is really better to have in user space something like (as Dan noted)
> 
> If it is possible to designate the memory as standby or online in the
> s390 admin interface and the kernel does have access to this
> information it makes sense to forward it to userspace (as separate
> s390-specific property). If not then you need to make some kind of
> assumption like below and the user can tune the script according to
> their usecase.

Also true, standby memory really represents a distinct type of memory
block (memory seems to be there but really isn't). Right now I am
thinking about something like this (tried to formulate it on a very
generic level because we can't predict which mechanism might want to
make use of these types in the future).


/*
 * Memory block types allow user space to formulate rules if and how to
 * online memory blocks. The types are exposed to user space as text
 * strings in sysfs. While the typical online strategies are described
 * along with the types, there are use cases where that can differ (e.g.
 * use MOVABLE zone for more reliable huge page usage, use NORMAL zone
 * due to zone imbalance or because memory unplug is not intended).
 *
 * MEMORY_BLOCK_NONE:
 *  No memory block is to be created (e.g. device memory). Used internally
 *  only.
 *
 * MEMORY_BLOCK_REMOVABLE:
 *  This memory block type should be treated as if it can be
 *  removed/unplugged from the system again. E.g. there is a hardware
 *  interface to unplug such memory. This memory block type is usually
 *  onlined to the MOVABLE zone, to e.g. make offlining of it more
 *  reliable. Examples include ACPI and PPC DIMMs.
 *
 * MEMORY_BLOCK_UNREMOVABLE:
 *  This memory block type should be treated as if it can not be
 *  removed/unplugged again. E.g. there is no hardware interface to
 *  unplug such memory. This memory block type is usually onlined to
 *  the NORMAL zone, as offlining is not beneficial. Examples include boot
 *  memory on most architectures and memory added via balloon devices.
 *
 * MEMORY_BLOCK_STANDBY:
 *  The memory block type should be treated as if it can be
 *  removed/unplugged again, however the actual memory hot(un)plug is
 *  performed by onlining/offlining. In virtual environments, such memory
 *  is usually added during boot and never removed. Onlining memory will
 *  result in memory getting allocated to a VM. This memory type is usually
 *  not onlined automatically but explicitly by the administrator. One
 *  example is standby memory on s390x.
 */

> 
>>
>> if (isS390x() && type == "dimm") {
>>      /* don't online, on s390x system DIMMs are standby memory */
>> }
> 
> Thanks
> 
> Michal
> 


-- 

Thanks,

David / dhildenb

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