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

[Alexander Duyck]

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.

That also reminds me that one thing this patch set still doesn't
address is what do we do about a direct assigned device or some other
form of shared memory where we want to keep the virtual mapping
beneath the guest pinned to a given set of physical memory.