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

[Alexander Duyck]

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.

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.

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

Thanks.

- Alex