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

Vlastimil Babka
2020-09-24T13:59:55Z

On 9/23/20 5:26 PM, David Hildenbrand wrote:
> On 23.09.20 16:31, Vlastimil Babka wrote:
>> On 9/16/20 9:31 PM, David Hildenbrand wrote:
>>
>
> 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.

Mel already explained that, so I won't repeat.

> 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

Sure, but then it can become a pointless code churn.

> I'd like to stress that what I propose here is both simple and powerful.
>
>> possible I think, such as preparing a larger MIGRATE_UNMOVABLE area in the
>> existing memory before we allocate those long-term management structures. Or
>> onlining a bunch of blocks as zone_movable first and only later convert to
>> zone_normal in a controlled way when existing normal zone becomes depeted?
>
> I see the following (more or less complicated) alternatives
>
> 1) Having a larger MIGRATE_UNMOVABLE area
>
> a) Sizing it is difficult. I mean you would have to plan ahead for all
> memory you might eventually hotplug later - and that could even be

Yeah, hence my worry about existing interfaces that work on 128MB blocks
individually without a larger strategy.

> impossible if you hotplug quite a lot of memory to a smaller machine.
> (I've seen people in the vm/container world trying to hotplug 128GB
> DIMMs to 2GB VMs ... and failing for obvious reasons)

Some planning should still be possible to maximize the contiguous area without
unmovable allocations.

> b) not really desired. You usually want to have most memory movable, not
> the opposite (just because you might hotplug memory in small chunks later).
>
> 2) smarter onlining
>
> I have prototype patches for better auto-onlining (which I'll share at
> some point), where I balance between ZONE_NORMAL and ZONE_MOVABLE in a
> defined ratio. Assuming something very simple, adding separate memory
> blocks and onlining them based on the current zone ratio (assuming a 1:4
> normal:movable target ratio) would (without some other policies I have
> in place) result in something like this for hotplugged memory (via
> virtio-mem):
>
> [N][M][M][M][M][N][M][M][M][M][N][M][M][M][M]...
>
> (note: layout is suboptimal, just a simple example)
>
> But even here, all [N] memory blocks would immediately be use for
> allocations for the memmap of successive blocks. It doesn't solve the
> dependency issues.
>
> Now assume we would want to group [N] in a way to allow for gigantic
> pages, like
>
> [N][N][N][N][N][N][N][N][M][M][M][M] ....
>
> we would, once again, never be able to allocate a gigantic page because
> all [N] would contain a memmap.

The second approach should work, if you know how much you are going to online,
and plan the size the N group accordingly, and if the onlined amount is several
gigabytes, then only the first one (or first X) will be unusable for a gigantic
page, but the rest would be? Can't get much better than that.

> 3) conversion from MOVABLE -> NORMAL
>
> While a conversion from MOVABLE to NORMAL would be interesting to see,
> it's going to be a challenging task to actually implement (people expect
> that page_zone() remains stable). Without any hacks, we'd have to
>
> 1. offline the selected (MOVABLE) memory block/chunk
> 2. online the selected memory block/chunk to the NORMAL zone
>
> This is not something we can do out of random context (for example, we
> need both, the device hotplug lock and the memory hotplug lock, as we
> might race with user space) - so there might still be a chance of
> corner-case OOMs.

Right, it's trickier than I thought.

> (I assume there could also be quite a negative performance impact when
> always relying on the conversion, and not properly planning ahead as in 2.)
>
>>
>> I guess it's an issue that the e.g. 128M block onlines are so disconnected from
>> each other it's hard to employ a strategy that works best for e.g. a whole bunch
>> of GB onlined at once. But I noticed some effort towards new API, so maybe that
>> will be solved there too?
>
> While new interfaces might make it easier to identify boundaries of
> separate DIMMs (e.g., to online a single DIMM either movable or
> unmovable - which can partially be done right now when going via memory
> resource boundaries), it doesn't help for the use case of adding
> separate memory blocks.
>
> So while having an automatic conversion from MOVABLE -> NORMAL would be
> interesting, I doubt we'll see it in the foreseeable future. Are there
> any similarly simple alternatives to optimize this?

I've reviewed the series and I won't block it - yes it's an optimistic approach
that can break and leave us with code churn. But at least it's not that much
code and the extra test in __free_one_page() shouldn't make this hotpath too
worse. But I still hope we can achieve a more robust solution one day.

> Thanks!
>