Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation

[David Hildenbrand <david@xxxxxxxxxx>]

On 24.09.20 11:40, Mel Gorman wrote:
> On Wed, Sep 23, 2020 at 05:26:06PM +0200, David Hildenbrand wrote:
>>>>> ???On 2020-09-16 20:34, David Hildenbrand wrote:
>>>>>> When adding separate memory blocks via add_memory*() and onlining them
>>>>>> immediately, the metadata (especially the memmap) of the next block will 
>>>>>> be
>>>>>> placed onto one of the just added+onlined block. This creates a chain
>>>>>> of unmovable allocations: If the last memory block cannot get
>>>>>> offlined+removed() so will all dependant ones. We directly have unmovable
>>>>>> allocations all over the place.
>>>>>> This can be observed quite easily using virtio-mem, however, it can also
>>>>>> be observed when using DIMMs. The freshly onlined pages will usually be
>>>>>> placed to the head of the freelists, meaning they will be allocated next,
>>>>>> turning the just-added memory usually immediately un-removable. The
>>>>>> fresh pages are cold, prefering to allocate others (that might be hot)
>>>>>> also feels to be the natural thing to do.
>>>>>> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when
>>>>>> adding separate, successive memory blocks, each memory block will have
>>>>>> unmovable allocations on them - for example gigantic pages will fail to
>>>>>> allocate.
>>>>>> While the ZONE_NORMAL doesn't provide any guarantees that memory can get
>>>>>> offlined+removed again (any kind of fragmentation with unmovable
>>>>>> allocations is possible), there are many scenarios (hotplugging a lot of
>>>>>> memory, running workload, hotunplug some memory/as much as possible) 
>>>>>> where
>>>>>> we can offline+remove quite a lot with this patchset.
>>>>>
>>>>> Hi David,
>>>>>
>>>>
>>>> Hi Oscar.
>>>>
>>>>> I did not read through the patchset yet, so sorry if the question is 
>>>>> nonsense, but is this not trying to fix the same issue the vmemmap 
>>>>> patches did? [1]
>>>>
>>>> Not nonesense at all. It only helps to some degree, though. It solves the 
>>>> dependencies due to the memmap. However, it???s not completely ideal, 
>>>> especially for single memory blocks.
>>>>
>>>> With single memory blocks (virtio-mem, xen-balloon, hv balloon, ppc dlpar) 
>>>> you still have unmovable (vmemmap chunks) all over the physical address 
>>>> space. Consider the gigantic page example after hotplug. You directly 
>>>> fragmented all hotplugged memory.
>>>>
>>>> Of course, there might be (less extreme) dependencies due page tables for 
>>>> the identity mapping, extended struct pages and similar.
>>>>
>>>> Having that said, there are other benefits when preferring other memory 
>>>> over just hotplugged memory. Think about adding+onlining memory during 
>>>> boot (dimms under QEMU, virtio-mem), once the system is up you will have 
>>>> most (all) of that memory completely untouched.
>>>>
>>>> So while vmemmap on hotplugged memory would tackle some part of the issue, 
>>>> there are cases where this approach is better, and there are even benefits 
>>>> when combining both.
>>>
>>
>> Hi Vlastimil,
>>
>>> I see the point, but I don't think the head/tail mechanism is great for 
>>> this. It
>>> might sort of work, but with other interfering activity there are no 
>>> guarantees
>>> and it relies on a subtle implementation detail. There are better mechanisms
>>
>> For the specified use case of adding+onlining a whole bunch of memory
>> this works just fine. We don't care too much about "other interfering
>> activity" as you mention here, or about guarantees - this is a pure
>> optimization that seems to work just fine in practice.
>>
>> I'm not sure about the "subtle implementation detail" - buddy merging,
>> and head/tail of buddy lists are a basic concept of our page allocator.
>> If that would ever change, the optimization here would be lost and we
>> would have to think of something else. Nothing would actually break -
>> and it's all kept directly in page_alloc.c
>>

Hi Mel,

thanks for your reply.

> 
> It's somewhat subtle because it's relying heavily on the exact ordering
> of how pages are pulled from the free lists at the moment. Lets say for
> example that someone was brave enough to tackle the problem of the giant
> zone lock and split the zone into allocation arenas (like what glibc does
> to split the lock). Depending on the exact ordering of how pages are
> added and removed from the list would break your approach. I'm wary of

First of all, it would not break it (as I already said). The
optimization would be lost. Totally acceptable.

However, I assume we would apply the same technique (optimized buddy
merging - placing to head/tail, page shuffling) on these allocation
arenas. So the optimization would still mostly apply, just in different
granularity - which would be fine.

> anything that relies on the ordering of freelists for correctness becauuse
> it limits the ability to fix the zone lock (which has been overdue for
> fixing for years now and getting worse as node sizes increase).

"for correctness" - no, this is an optimization. As I said, there are no
guarantees. Please keep that in mind.

(also, page shuffling relies on the ordering of freelists right now ...
for correctness)

> 
> To be robust, you'd need to do something like early memory bring-up whereby
> pages are directly allocated from one part of the DIMM (presumably the
> start) and use that for the metadata -- potentially all the metadata that
> would be necessary to plug/unplug the entire DIMM. This would effectively
> be unmovable but if you want to guarantee that all the memory except the
> metadata can be unplugged, you do not have much alteratives. Playing games
> with the ordering of the freelists will simply end up as "sometimes works,
> sometimes does not". 

As answered to Oscar already, while something like that might be
feasible for DIMMs in the future (and there are still quite some issues
to be sorted out), it isn't always desired adding separate (small -
e.g., 128MB) memory blocks.  You - again- have unmovable allocations all
over the place that won't allow you to allocate any gigantic page.

> 
> In terms of forcing ranges to be UNMOVABLE or MOVABLE (either via zones
> or by implementing "sticky" pageblocks which hits complex reclaim-related
> problems), you start running into problems similar to lowmem starvation
> where a page cache allocation fails because unmovable metadata cannot
> be allocated.

Exactly.

> 
> I suggest you keep it simple -- statically allocate the potential
> metadata needed in the future even though it limits the maximum amount
> of memory that can be unplugged. The alternative is unpredictable
> plug/unplug success rates.
> 

I'm sorry I can't follow. How is this "simple"?  Or even "simpler" than
what I suggest?

And as I said, it doesn't always work. Assume I hotplug 128GB to a 2GB
machine via virtio-mem (which works just fine, as we add+online memory
in small chunks compared to a single, huge DIMM), I would have to
pre-allocate 2GB just for the memmap - which obviously doesn't work.

Again, I'd like to stress that this is a pure optimization that I am
proposing - nothing would "break" when ripping it out again, except that
we lose the optimizations I mentioned.

-- 
Thanks,

David / dhildenb