Re: [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages

[David Hildenbrand]

On 13.03.19 23:54, Alexander Duyck wrote:
> On Wed, Mar 13, 2019 at 9:39 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 13.03.19 17:37, Alexander Duyck wrote:
>>> On Wed, Mar 13, 2019 at 5:18 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>
>>>> On 13.03.19 12:54, Nitesh Narayan Lal wrote:
>>>>>
>>>>> On 3/12/19 5:13 PM, Alexander Duyck wrote:
>>>>>> On Tue, Mar 12, 2019 at 12:46 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>>>>>>> On 3/8/19 4:39 PM, Alexander Duyck wrote:
>>>>>>>> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>>>>>>>>> On 3/8/19 2:25 PM, Alexander Duyck wrote:
>>>>>>>>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>>>>>>>>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote:
>>>>>>>>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>>>>>>>>>>>>> On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote:
>>>>>>>>>>>>>> The only other thing I still want to try and see if I can do is to add
>>>>>>>>>>>>>> a jiffies value to the page private data in the case of the buddy
>>>>>>>>>>>>>> pages.
>>>>>>>>>>>>> Actually there's one extra thing I think we should do, and that is make
>>>>>>>>>>>>> sure we do not leave less than X% off the free memory at a time.
>>>>>>>>>>>>> This way chances of triggering an OOM are lower.
>>>>>>>>>>>> If nothing else we could probably look at doing a watermark of some
>>>>>>>>>>>> sort so we have to have X amount of memory free but not hinted before
>>>>>>>>>>>> we will start providing the hints. It would just be a matter of
>>>>>>>>>>>> tracking how much memory we have hinted on versus the amount of memory
>>>>>>>>>>>> that has been pulled from that pool.
>>>>>>>>>>> This is to avoid false OOM in the guest?
>>>>>>>>>> Partially, though it would still be possible. Basically it would just
>>>>>>>>>> be a way of determining when we have hinted "enough". Basically it
>>>>>>>>>> doesn't do us much good to be hinting on free memory if the guest is
>>>>>>>>>> already constrained and just going to reallocate the memory shortly
>>>>>>>>>> after we hinted on it. The idea is with a watermark we can avoid
>>>>>>>>>> hinting until we start having pages that are actually going to stay
>>>>>>>>>> free for a while.
>>>>>>>>>>
>>>>>>>>>>>>  It is another reason why we
>>>>>>>>>>>> probably want a bit in the buddy pages somewhere to indicate if a page
>>>>>>>>>>>> has been hinted or not as we can then use that to determine if we have
>>>>>>>>>>>> to account for it in the statistics.
>>>>>>>>>>> The one benefit which I can see of having an explicit bit is that it
>>>>>>>>>>> will help us to have a single hook away from the hot path within buddy
>>>>>>>>>>> merging code (just like your arch_merge_page) and still avoid duplicate
>>>>>>>>>>> hints while releasing pages.
>>>>>>>>>>>
>>>>>>>>>>> I still have to check PG_idle and PG_young which you mentioned but I
>>>>>>>>>>> don't think we can reuse any existing bits.
>>>>>>>>>> Those are bits that are already there for 64b. I think those exist in
>>>>>>>>>> the page extension for 32b systems. If I am not mistaken they are only
>>>>>>>>>> used in VMA mapped memory. What I was getting at is that those are the
>>>>>>>>>> bits we could think about reusing.
>>>>>>>>>>
>>>>>>>>>>> If we really want to have something like a watermark, then can't we use
>>>>>>>>>>> zone->free_pages before isolating to see how many free pages are there
>>>>>>>>>>> and put a threshold on it? (__isolate_free_page() does a similar thing
>>>>>>>>>>> but it does that on per request basis).
>>>>>>>>>> Right. That is only part of it though since that tells you how many
>>>>>>>>>> free pages are there. But how many of those free pages are hinted?
>>>>>>>>>> That is the part we would need to track separately and then then
>>>>>>>>>> compare to free_pages to determine if we need to start hinting on more
>>>>>>>>>> memory or not.
>>>>>>>>> Only pages which are isolated will be hinted, and once a page is
>>>>>>>>> isolated it will not be counted in the zone free pages.
>>>>>>>>> Feel free to correct me if I am wrong.
>>>>>>>> You are correct up to here. When we isolate the page it isn't counted
>>>>>>>> against the free pages. However after we complete the hint we end up
>>>>>>>> taking it out of isolation and returning it to the "free" state, so it
>>>>>>>> will be counted against the free pages.
>>>>>>>>
>>>>>>>>> If I am understanding it correctly you only want to hint the idle pages,
>>>>>>>>> is that right?
>>>>>>>> Getting back to the ideas from our earlier discussion, we had 3 stages
>>>>>>>> for things. Free but not hinted, isolated due to hinting, and free and
>>>>>>>> hinted. So what we would need to do is identify the size of the first
>>>>>>>> pool that is free and not hinted by knowing the total number of free
>>>>>>>> pages, and then subtract the size of the pages that are hinted and
>>>>>>>> still free.
>>>>>>> To summarize, for now, I think it makes sense to stick with the current
>>>>>>> approach as this way we can avoid any locking in the allocation path and
>>>>>>> reduce the number of hypercalls for a bunch of MAX_ORDER - 1 page.
>>>>>> I'm not sure what you are talking about by "avoid any locking in the
>>>>>> allocation path". Are you talking about the spin on idle bit, if so
>>>>>> then yes.
>>>>> Yeap!
>>>>>> However I have been testing your patches and I was correct
>>>>>> in the assumption that you forgot to handle the zone lock when you
>>>>>> were freeing __free_one_page.
>>>>> Yes, these are the steps other than the comments you provided in the
>>>>> code. (One of them is to fix release_buddy_page())
>>>>>>  I just did a quick copy/paste from your
>>>>>> zone lock handling from the guest_free_page_hinting function into the
>>>>>> release_buddy_pages function and then I was able to enable multiple
>>>>>> CPUs without any issues.
>>>>>>
>>>>>>> For the next step other than the comments received in the code and what
>>>>>>> I mentioned in the cover email, I would like to do the following:
>>>>>>> 1. Explore the watermark idea suggested by Alex and bring down memhog
>>>>>>> execution time if possible.
>>>>>> So there are a few things that are hurting us on the memhog test:
>>>>>> 1. The current QEMU patch is only madvising 4K pages at a time, this
>>>>>> is disabling THP and hurts the test.
>>>>> Makes sense, thanks for pointing this out.
>>>>>>
>>>>>> 2. The fact that we madvise the pages away makes it so that we have to
>>>>>> fault the page back in in order to use it for the memhog test. In
>>>>>> order to avoid that penalty we may want to see if we can introduce
>>>>>> some sort of "timeout" on the pages so that we are only hinting away
>>>>>> old pages that have not been used for some period of time.
>>>>>
>>>>> Possibly using MADVISE_FREE should also help in this, I will try this as
>>>>> well.
>>>>
>>>> I was asking myself some time ago how MADVISE_FREE will be handled in
>>>> case of THP. Please let me know your findings :)
>>>
>>> The problem with MADVISE_FREE is that it will add additional
>>> complication to the QEMU portion of all this as it only applies to
>>> anonymous memory if I am not mistaken.
>>
>> Just as MADV_DONTNEED. So nothing new. Future work.
> 
> I'm pretty sure you can use MADV_DONTNEED to free up file backed
> memory, I don't believe this is the case for MADV_FREE, but maybe I am
> mistaken.

"MADV_DONTNEED cannot be applied to locked pages, Huge TLB pages, or
VM_PFNMAP pages."

For shmem, hugetlbfs and friends one has to use FALLOC_FL_PUNCH_HOLE as
far as I remember (e.g. QEMU postcopy migration has to use it).

So effectively, virtio-balloon can as of now only really deal with
anonymous memory. And it is the same case for free page hinting.

> 
> On a side note I was just reviewing some stuff related to the reserved
> bit and on-lining hotplug memory, and it just occurred to me that most
> the PG_offline bit would be a good means to indicate that we hinted
> away a page out of the buddy allocator, especially since it is already
> used by the balloon drivers anyway.  We would just have to add a call
> to make sure we clear it when we call __ClearPageBuddy. It looks like
> that would currently be in del_page_from_free_area, at least for
> linux-next.

Hmm, if we only knew who came up with PG_offline ... ;)

Unfortunately PG_offline is not a bit, it is mapcount value just like
PG_buddy. Well okay, it is a bit in the mapcount value - but as of now,
a page can only have one such "page type" at a time as far as I recall.

> 
> I just wanted to get your thoughts on that as it seems like it might
> be a good fit.

It would be if we could have multiple page types at a time. I haven't
had a look yet how realistic that would be. As you correctly noted,
balloon drivers use that bit as of now to mark pages that are logically
offline (here: "inflated").

> 
> Thanks.
> 
> - Alex
> 


-- 

Thanks,

David / dhildenb