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

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 3/8/19 4:39 PM, Alexander Duyck wrote:
> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>> On 3/8/19 2:25 PM, Alexander Duyck wrote:
>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote:
>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@redhat.com> 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.
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.
2. Benchmark hinting v/s non-hinting more extensively.
Let me know if you have any specific suggestions in terms of the tools I
can run to do the same. (I am planning to run atleast netperf, hackbench
and stress for this).

>
>>>>>>> With that we could track the age of the page so it becomes
>>>>>>> easier to only target pages that are truly going cold rather than
>>>>>>> trying to grab pages that were added to the freelist recently.
>>>>>> I like that but I have a vague memory of discussing this with Rik van
>>>>>> Riel and him saying it's actually better to take away recently used
>>>>>> ones. Can't see why would that be but maybe I remember wrong. Rik - am I
>>>>>> just confused?
>>>>> It is probably to cut down on the need for disk writes in the case of
>>>>> swap. If that is the case it ends up being a trade off.
>>>>>
>>>>> The sooner we hint the less likely it is that we will need to write a
>>>>> given page to disk. However the sooner we hint, the more likely it is
>>>>> we will need to trigger a page fault and pull back in a zero page to
>>>>> populate the last page we were working on. The sweet spot will be that
>>>>> period of time that is somewhere in between so we don't trigger
>>>>> unnecessary page faults and we don't need to perform additional swap
>>>>> reads/writes.
>>>> --
>>>> Regards
>>>> Nitesh
>>>>
>> --
>> Regards
>> Nitesh
>>
-- 
Regards
Nitesh


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