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

[Nitesh Narayan Lal]

On 3/13/19 8:17 AM, David Hildenbrand 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 :)
I will do that.
If we don't end up finding any appropriate page flag to track the age of
free page. I am wondering if I can somehow use bitmap to track the free
count for each PFN.
>
-- 
Regards
Nitesh

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