On Tue, Mar 10, 2020 at 08:13:19PM +0100, David Hildenbrand wrote:
> On 10.03.20 20:05, David Rientjes wrote:
> > On Tue, 10 Mar 2020, David Hildenbrand wrote:
> >
> >> Commit 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> >> changed the behavior when deflation happens automatically. Instead of
> >> deflating when called by the OOM handler, the shrinker is used.
> >>
> >> However, the balloon is not simply some other slab cache that should be
> >> shrunk when under memory pressure. The shrinker does not have a concept of
> >> priorities yet, so this behavior cannot be configured. Eventually once
> >> that is in place, we might want to switch back after doing proper
> >> testing.
> >>
> >> There was a report that this results in undesired side effects when
> >> inflating the balloon to shrink the page cache. [1]
> >> "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."
> >>
> >> The name "deflate on OOM" makes it pretty clear when deflation should
> >> happen - after other approaches to reclaim memory failed, not while
> >> reclaiming. This allows to minimize the footprint of a guest - memory
> >> will only be taken out of the balloon when really needed.
> >>
> >> Keep using the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT, because
> >> this has no such side effects. Always register the shrinker with
> >> VIRTIO_BALLOON_F_FREE_PAGE_HINT now. We are always allowed to reuse free
> >> pages that are still to be processed by the guest. The hypervisor takes
> >> care of identifying and resolving possible races between processing a
> >> hinting request and the guest reusing a page.
> >>
> >> In contrast to pre commit 71994620bb25 ("virtio_balloon: replace oom
> >> notifier with shrinker"), don't add a moodule parameter to configure the
> >> number of pages to deflate on OOM. Can be re-added if really needed.
> >> Also, pay attention that leak_balloon() returns the number of 4k pages -
> >> convert it properly in virtio_balloon_oom_notify().
> >>
> >> Testing done by Tyler for future reference:
> >> Test setup: VM with 16 CPU, 64GB RAM. Running Debian 10. We have a 42
> >> GB file full of random bytes that we continually cat to /dev/null.
> >> This fills the page cache as the file is read. Meanwhile we trigger
> >> the balloon to inflate, with a target size of 53 GB. This setup causes
> >> the balloon inflation to pressure the page cache as the page cache is
> >> also trying to grow. Afterwards we shrink the balloon back to zero (so
> >> total deflate = total inflate).
> >>
> >> Without patch (kernel 4.19.0-5):
> >> Inflation never reaches the target until we stop the "cat file >
> >> /dev/null" process. Total inflation time was 542 seconds. The longest
> >> period that made no net forward progress was 315 seconds (see attached
> >> graph).
> >> Result of "grep balloon /proc/vmstat" after the test:
> >> balloon_inflate 154828377
> >> balloon_deflate 154828377
> >>
> >> With patch (kernel 5.6.0-rc4+):
> >> Total inflation duration was 63 seconds. No deflate-queue activity
> >> occurs when pressuring the page-cache.
> >> Result of "grep balloon /proc/vmstat" after the test:
> >> balloon_inflate 12968539
> >> balloon_deflate 12968539
> >>
> >> Conclusion: This patch fixes the issue. In the test it reduced
> >> inflate/deflate activity by 12x, and reduced inflation time by 8.6x.
> >> But more importantly, if we hadn't killed the "grep balloon
> >> /proc/vmstat" process then, without the patch, the inflation process
> >> would never reach the target.
> >>
> >> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
> >>
> >> Reported-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> >> Tested-by: Tyler Sanderson <tysand@xxxxxxxxxx>
> >> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> >> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
> >> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> >> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >> Cc: Nadav Amit <namit@xxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> >
>
> Thanks!
David could you repost with corrected commit log
so Andrew can merge? To make sure it's not missed,
it's probably a good idea to have a cover letter
mentioning mm in the subject.
> > Should this have:
> >
> > Cc: stable@xxxxxxxxxxxxxxx # 4.19+
>
> I guess as nothing will actually "crash" it's not worth stable.
>
> --
> Thanks,
>
> David / dhildenb
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