On 30.01.20 16:20, Michael S. Tsirkin wrote:

On Thu, Jan 30, 2020 at 04:02:34PM +0100, David Hildenbrand wrote:

quoted

On 29.01.20 20:11, Tyler Sanderson wrote:

quoted

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/71994620bb25a8b109388fefa9e99a28e355255a#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.
    >
    > If file IO is ongoing during this balloon inflation then the page
    cache
    > could be growing which further puts "back pressure" on the balloon
    > trying to inflate. In testing I've seen periods of > 45 seconds where
    > balloon inflation makes no net forward progress.
    >
    > This wasn't a problem before the change to the shrinker API since
    forced
    > balloon deflation only occurred via the OOM notifier callback
    which was
    > invoked only after the page cache had depleted.
    >
    > Is this new busy behavior working as intended?

    Please note that the shrinker will only be registered in case we have
    VIRTIO_BALLOON_F_DEFLATE_ON_OOM - (which is AFAIK very rare) - to
    implement automatic balloon deflation when the guest is under memory
    pressure.


    Are you actually experiencing issues with that or did you just stumble
    over the code?


We have a use case that is encountering this (and that registers
DEFLATE_ON_OOM). We can work around this, but it does seem inefficient.
I understand there were good reasons for moving away from the OOM
notifier callback, but I'm wondering if the balloon driver could specify
a "nice" level to the shrinker API that would cause it to be reclaimed
from only as a last resort?
 

Cc-ing linux-mm, Michal and Michael.

Interesting.  VIRTIO_BALLOON_F_DEFLATE_ON_OOM is really
underspecified in a bunch of ways.

I'll wait to see what does Michal say from Linux POV.

Just wondering, does implying that we are using the shrinker that a

echo 3 > /proc/sys/vm/drop_caches

Will deflate the whole balloon? If yes, than that's *really* not desired.

-- 
Thanks,

David / dhildenb

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