Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

[David Hildenbrand]

On 22.01.20 17:46, Michal Hocko wrote:
> On Wed 22-01-20 12:58:16, David Hildenbrand wrote:
>> On 22.01.20 11:54, David Hildenbrand wrote:
>>> On 22.01.20 11:42, Michal Hocko wrote:
>>>> On Wed 22-01-20 11:39:08, David Hildenbrand wrote:
>>>>>>>> Really, the interface is flawed and should have never been merged in the
>>>>>>>> first place. We cannot simply remove it altogether I am afraid so let's
>>>>>>>> at least remove the bogus code and pretend that the world is a better
>>>>>>>> place where everything is removable except the reality sucks...
>>>>>>>
>>>>>>> As I expressed already, the interface works as designed/documented and
>>>>>>> has been used like that for years.
>>>>>>
>>>>>> It seems we do differ in the usefulness though. Using a crappy interface
>>>>>> for years doesn't make it less crappy. I do realize we cannot remove the
>>>>>> interface but we can remove issues with the implementation and I dare to
>>>>>> say that most existing users wouldn't really notice.
>>>>>
>>>>> Well, at least powerpc-utils (why this interface was introduced) will
>>>>> notice a) performance wise and b) because more logging output will be
>>>>> generated (obviously non-offlineable blocks will be tried to offline).
>>>>
>>>> I would really appreciate some specific example for a real usecase. I am
>>>> not familiar with powerpc-utils worklflows myself.
>>>>
>>>
>>> Not an expert myself:
>>>
>>> https://github.com/ibm-power-utilities/powerpc-utils
>>>
>>> -> src/drmgr/drslot_chrp_mem.c
>>>
>>> On request to remove some memory it will
>>>
>>> a) Read "->removable" of all memory blocks ("lmb")
>>> b) Check if the request can be fulfilled using the removable blocks
>>> c) Try to offline the memory blocks by trying to offline it. If that
>>> succeeded, trigger removeal of it using some hypervisor hooks.
>>>
>>> Interestingly, with "AMS ballooning", it will already consider the
>>> "removable" information useless (most probably, because of
>>> non-migratable balloon pages that can be offlined - I assume the powerpc
>>> code that I converted to proper balloon compaction just recently). a)
>>> and b) is skipped.
>>>
>>> Returning "yes" on all blocks will make them handle it just like if "AMS
>>> ballooning" is active. So any memory block will be tried. Should work
>>> but will be slower if no ballooning is active.
>>>
>>
>> On lsmem:
>>
>> https://www.ibm.com/support/knowledgecenter/linuxonibm/com.ibm.linux.z.lgdd/lgdd_r_lsmem_cmd.html
>>
>> "
>> Removable
>>     yes if the memory range can be set offline, no if it cannot be set
>> offline. A dash (-) means that the range is already offline. The kernel
>> method that identifies removable memory ranges is heuristic and not
>> exact. Occasionally, memory ranges are falsely reported as removable or
>> falsely reported as not removable.
>> "
>>
>> Usage of lsmem paird with chmem:
>>
>> https://access.redhat.com/solutions/3937181
>>
>>
>> Especially interesting for IBM z Systems, whereby memory
>> onlining/offlining will trigger the actual population of memory in the
>> hypervisor. So if an admin wants to offline some memory (to give it back
>> to the hypervisor), it would use lsmem to identify such blocks first,
>> instead of trying random blocks until one offlining request succeeds.
> 
> I am sorry for being dense here but I still do not understand why s390

It's good that we talk about it :) It's hard to reconstruct actual use
cases from tools and some documentation only ...

Side note (just FYI): One difference on s390x compared to other
architectures (AFAIKS) is that once memory is offline, you might not be
allowed (by the hypervisor) to online it again - because it was
effectively unplugged. Such memory is not removed via remove_memory(),
it's simply kept offline.


> and the way how it does the hotremove matters here. Afterall there are
> no arch specific operations done until the memory is offlined. Also
> randomly checking memory blocks and then hoping that the offline will
> succeed is not way much different from just trying the offline the
> block. Both have to crawl through the pfn range and bail out on the
> unmovable memory.

I think in general we have to approaches to memory unplugging.

1. Know explicitly what you want to unplug (e.g., a DIMM spanning
multiple memory blocks).

2. Find random memory blocks you can offline/unplug.


For 1, I think we both agree that we don't need this. Just try to
offline and you know if it worked.

Now of course, for 2 you can try random blocks until you succeeded. From
a sysadmin point of view that's very inefficient. From a powerpc-utils
point of view, that's inefficient.

I learned just now, "chmem"[1] has a mode where you can specify a "size"
and not only a range. So a sysadmin can still control onlining/offlining
for this use case with a few commands. In other tools (e.g.,
powerpc-utils), well, you have to try to offline random memory blocks
(just like chmem does).


AFAIK, once we turn /sys/.../removable useless, I can see the following
changes:

1. Trying to offline a certain amount of memory blocks gets slower/takes
longer/is less efficient. Might be tolerable. The tools seem to keep
working.

2. You can no longer make a rough estimate how much memory you could
offline - before you actually try to offline it. I can only imagine that
something like this makes sense in a virtual environment (e.g., IBM z)
to balance memory between virtual machines, but I am not aware of a real
user of something like that.


So what I can do is

a) Come up with a patch that rips that stuff out (well I have that
already lying around)

b) Describe the existing users + changes we will see

c) CC relevant people I identify (lsmem/chmem/powerpc-utils/etc.) on the
patch to see if we are missing other use cases/users/implications.

Sounds like a plan?


[1]
https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/chmem.c

-- 
Thanks,

David / dhildenb