On Mon, Feb 03, 2020 at 12:32:05PM -0800, Tyler Sanderson wrote:

There were apparently good reasons for moving away from OOM notifier callback:
https://lkml.org/lkml/2018/7/12/314
https://lkml.org/lkml/2018/8/2/322

In particular the OOM notifier is worse than the shrinker because:

 1. It is last-resort, which means the system has already gone through heroics
    to prevent OOM. Those heroic reclaim efforts are expensive and impact
    application performance.
 2. It lacks understanding of NUMA or other OOM constraints.
 3. It has a higher potential for bugs due to the subtlety of the callback
    context.

Given the above, I think the shrinker API certainly makes the most sense _if_
the balloon size is static. In that case memory should be reclaimed from the
balloon early and proportionally to balloon size, which the shrinker API
achieves.

OK that sounds like VIRTIO_BALLOON_F_FREE_PAGE_HINT then.

However, if the balloon is inflating and intentionally causing memory pressure
then this results in the inefficiency pointed out earlier.

And that sounds like VIRTIO_BALLOON_F_DEFLATE_ON_OOM.

If the balloon is inflating but not causing memory pressure then there is no
problem with either API.

This suggests another route: rather than cause memory pressure to shrink the
page cache, the balloon could issue the equivalent of "echo 3 > /proc/sys/vm/
drop_caches".
Of course ideally, we want to be more fine grained than "drop everything". We
really want an API that says "drop everything that hasn't been accessed in the
last 5 minutes".

This would eliminate the need for the balloon to cause memory pressure at all
which avoids the inefficiency in question. Furthermore, this pairs nicely with
the FREE_PAGE_HINT feature.

Well we still do have a regression. So we probably should revert
for now, and separately look for better solutions.

On Mon, Feb 3, 2020 at 9:04 AM Michael S. Tsirkin [off-list ref] wrote:

    On Mon, Feb 03, 2020 at 05:34:20PM +0100, David Hildenbrand wrote:
    > On 03.02.20 17:18, Alexander Duyck wrote:
    > > On Mon, 2020-02-03 at 08:11 -0500, Michael S. Tsirkin wrote:
    > >> On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote:
    > >>>
    > >>> On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W [off-list ref]
    wrote:
    > >>>
    > >>>     On Thursday, January 30, 2020 11:03 PM, David Hildenbrand wrote:
    > >>>     > On 29.01.20 20:11, Tyler Sanderson wrote:
    > >>>     > >
    > >>>     > >
    > >>>     > > On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <
    david@redhat.com
    > >>>     > > <mailto:david@redhat.com>> wrote:
    > >>>     > >
    > >>>     > >     On 29.01.20 01:22, Tyler Sanderson via Virtualization
    wrote:
    > >>>     > >     > A primary advantage of virtio balloon over other memory
    reclaim
    > >>>     > >     > mechanisms is that it can pressure the guest's page
    cache into
    > >>>     > >     shrinking.
    > >>>     > >     >
    > >>>     > >     > However, since the balloon driver changed to using the
    shrinker
    > >>>     API
    > >>>     > >     >
    > >>>     > >
    > >>>     > <https://github.com/torvalds/linux/commit/
    71994620bb25a8b109388fefa9
    > >>>     > e99a28e355255a#diff-fd202acf694d9eba19c8c64da3e480c9> this
    > >>>     > >     > use case has become a bit more tricky. I'm wondering
    what the
    > >>>     > intended
    > >>>     > >     > device implementation is.
    > >>>     > >     >
    > >>>     > >     > When inflating the balloon against page cache (i.e. no
    free
    > >>>     memory
    > >>>     > >     > remains) vmscan.c will both shrink page cache, but also
    invoke
    > >>>     the
    > >>>     > >     > shrinkers -- including the balloon's shrinker. So the
    balloon
    > >>>     driver
    > >>>     > >     > allocates memory which requires reclaim, vmscan gets
    this memory
    > >>>     > by
    > >>>     > >     > shrinking the balloon, and then the driver adds the
    memory back
    > >>>     to
    > >>>     > the
    > >>>     > >     > balloon. Basically a busy no-op.
    > >>>
    > >>>     Per my understanding, the balloon allocation won’t invoke
    shrinker as
    > >>>     __GFP_DIRECT_RECLAIM isn't set, no?
    > >>>
    > >>> I could be wrong about the mechanism, but the device sees lots of
    activity on
    > >>> the deflate queue. The balloon is being shrunk. And this only starts
    once all
    > >>> free memory is depleted and we're inflating into page cache.
    > >>
    > >> So given this looks like a regression, maybe we should revert the
    > >> patch in question 71994620bb25 ("virtio_balloon: replace oom notifier
    with shrinker")
    > >> Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT
    > >> shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't nice
    > >> at all.
    > >>
    > >> So it looks like all this rework introduced more issues than it
    > >> addressed ...
    > >>
    > >> I also CC Alex Duyck for an opinion on this.
    > >> Alex, what do you use to put pressure on page cache?
    > >
    > > I would say reverting probably makes sense. I'm not sure there is much
    > > value to having a shrinker running deflation when you are actively
    trying
    > > to increase the balloon. It would make more sense to wait until you are
    > > actually about to start hitting oom.
    >
    > I think the shrinker makes sense for free page hinting feature
    > (everything on free_page_list).
    >
    > So instead of only reverting, I think we should split it up and always
    > register the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT and the OOM
    > notifier (as before) for VIRTIO_BALLOON_F_MUST_TELL_HOST.

    OK ... I guess that means we need to fix shrinker to take
    VIRTIO_BALLOON_F_MUST_TELL_HOST into account correctly.
    Hosts ignore it at the moment but it's a fragile thing
    to do what it does and ignore used buffers.

    > (Of course, adapting what is being done in the shrinker and in the OOM
    > notifier)
    >
    > --
    > Thanks,
    >
    > David / dhildenb

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