Re: [PATCH v34 0/4] Virtio-balloon: support free page reporting

[David Hildenbrand]

On 29.06.2018 13:31, Wei Wang wrote:
> On 06/29/2018 03:46 PM, David Hildenbrand wrote:
>>>
>>> I'm afraid it can't. For example, when we have a guest booted, without
>>> too many memory activities. Assume the guest has 8GB free memory. The
>>> arch_free_page there won't be able to capture the 8GB free pages since
>>> there is no free() called. This results in no free pages reported to host.
>>
>> So, it takes some time from when the guest boots up until the balloon
>> device was initialized and therefore page hinting can start. For that
>> period, you won't get any arch_free_page()/page hinting callbacks, correct.
>>
>> However in the hypervisor, you can theoretically track which pages the
>> guest actually touched ("dirty"), so you already know "which pages were
>> never touched while booting up until virtio-balloon was brought to
>> life". These, you can directly exclude from migration. No interface
>> required.
>>
>> The remaining problem is pages that were touched ("allocated") by the
>> guest during bootup but freed again, before virtio-balloon came up. One
>> would have to measure how many pages these usually are, I would say it
>> would not be that many (because recently freed pages are likely to be
>> used again next for allocation). However, there are some pages not being
>> reported.
>>
>> During the lifetime of the guest, this should not be a problem,
>> eventually one of these pages would get allocated/freed again, so the
>> problem "solves itself over time". You are looking into the special case
>> of migrating the VM just after it has been started. But we have the
>> exact same problem also for ordinary free page hinting, so we should
>> rather solve that problem. It is not migration specific.
>>
>> If we are looking for an alternative to "problem solves itself",
>> something like "if virtio-balloon comes up, it will report all free
>> pages step by step using free page hinting, just like we would have from
>> "arch_free_pages()"". This would be the same interface we are using for
>> free page hinting - and it could even be made configurable in the guest.
>>
>> The current approach we are discussing internally for details about
>> Nitesh's work ("how the magic inside arch_fee_pages() will work
>> efficiently) would allow this as far as I can see just fine.
>>
>> There would be a tiny little window between virtio-balloon comes up and
>> it has reported all free pages step by step, but that can be considered
>> a very special corner case that I would argue is not worth it to be
>> optimized.
>>
>> If I am missing something important here, sorry in advance :)
>>
> 
> Probably I didn't explain that well. Please see my re-try:
> 
> That work is to monitor page allocation and free activities via 
> arch_alloc_pages and arch_free_pages. It has per-CPU lists to record the 
> pages that are freed to the mm free list, and the per-CPU lists dump the 
> recorded pages to a global list when any of them is full.
> So its own per-CPU list will only be able to get free pages when there 
> is an mm free() function gets called. If we have 8GB free memory on the 
> mm free list, but no application uses them and thus no mm free() calls 
> are made. In that case, the arch_free_pages isn't called, and no free 
> pages added to the per-CPU list, but we have 8G free memory right on the 
> mm free list.
> How would you guarantee the per-CPU lists have got all the free pages 
> that the mm free lists have?

As I said, if we have some mechanism that will scan the free pages (not
arch_free_page() once and report hints using the same mechanism step by
step (not your bulk interface)), this problem is solved. And as I said,
this is not a migration specific problem, we have the same problem in
the current page hinting RFC. These pages have to be reported.

> 
> - I'm also worried about the overhead of maintaining so many per-CPU 
> lists and the global list. For example, if we have applications 
> frequently allocate and free 4KB pages, and each per-CPU list needs to 
> implement the buddy algorithm to sort and merge neighbor pages. Today a 
> server can have more than 100 CPUs, then there will be more than 100 
> per-CPU lists which need to sync to a global list under a lock, I'm not 
> sure if this would scale well.

The overhead in the current RFC is definitely too high. But I consider
this a problem to be solved before page hinting would go upstream. And
we are discussing right now "if we have a reasonable page hinting
implementation, why would we need your interface in addition".

> 
> - This seems to be a burden imposed on the core mm memory 
> allocation/free path. The whole overhead needs to be carried during the 
> whole system life cycle. What we actually expected is to just make one 
> call to get the free page hints only when live migration happens.

You're focusing too much on the actual implementation of the page
hinting RFC right now. Assume for now that we would have
- efficient page hinting without degrading other CPUs and little
  overhead
- a mechanism that solves reporting free pages once after we started up
  virtio-balloon and actual free page hinting starts

Why would your suggestion still be applicable?

Your point for now is "I might not want to have page hinting enabled due
to the overhead, but still a live migration speedup". If that overhead
actually exists (we'll have to see) or there might be another reason to
disable page hinting, then we have to decide if that specific setup is
worth it merging your changes.

I am not (and don't want to be) in the position to make any decisions
here :) I just want to understand if two interfaces for free pages
actually make sense.

Maybe the argument "I don't want free page hinting" is good enough.

> 
> Best,
> Wei


-- 

Thanks,

David / dhildenb