On Friday, June 29, 2018 7:54 PM, David Hildenbrand wrote:

On 29.06.2018 13:31, Wei Wang wrote:

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On 06/29/2018 03:46 PM, David Hildenbrand wrote:

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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.

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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.

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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.

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- 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".

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- 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.

All the above "if we have", "assume we have" don't sound like a valid argument to me.
 

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.

I responded to Nitesh about the differences, you may want to check with him about this.
I would suggest you to send out your patches to LKML to get a discussion with the mm folks.

Best,
Wei