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

[David Hildenbrand]

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 :)

-- 

Thanks,

David / dhildenb