Re: [PATCH v3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

[David Hildenbrand]

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!

> Should this have:
> 
> Cc: stable@xxxxxxxxxxxxxxx # 4.19+

I guess as nothing will actually "crash" it's not worth stable.

-- 
Thanks,

David / dhildenb