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Date:   Wed, 11 Mar 2020 07:12:36 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, virtualization@lists.linux-foundation.org,
        Jason Wang <jasowang@redhat.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Tyler Sanderson <tysand@google.com>,
        Wei Wang <wei.w.wang@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Nadav Amit <namit@vmware.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v3] virtio-balloon: Switch back to OOM handler for
 VIRTIO_BALLOON_F_DEFLATE_ON_OOM
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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@google.com>
> >> Tested-by: Tyler Sanderson <tysand@google.com>
> >> Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> >> Cc: Michael S. Tsirkin <mst@redhat.com>
> >> Cc: Wei Wang <wei.w.wang@intel.com>
> >> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >> Cc: David Rientjes <rientjes@google.com>
> >> Cc: Nadav Amit <namit@vmware.com>
> >> Cc: Michal Hocko <mhocko@kernel.org>
> >> Cc: Andrew Morton <akpm@linux-foundation.org>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> > 
> > Acked-by: David Rientjes <rientjes@google.com>
> > 
> 
> 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@vger.kernel.org # 4.19+
> 
> I guess as nothing will actually "crash" it's not worth stable.
> 
> -- 
> Thanks,
> 
> David / dhildenb