> On Feb 3, 2020, at 2:50 PM, Nadav Amit <namit@xxxxxxxxxx> wrote: > >> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@xxxxxxxxxx> 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 <wei.w.wang@xxxxxxxxx> 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@xxxxxxxxxx >>>>>>> <mailto:david@xxxxxxxxxx>> 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. >> >> (Of course, adapting what is being done in the shrinker and in the OOM >> notifier) > > David, > > Please keep me posted. I decided to adapt the same solution as the virtio > balloon for the VMware balloon. If the verdict is that this is damaging and > the OOM notifier should be used instead, I will submit patches to move to > OOM notifier as well. Adding some information for the record (if someone googles this thread): In the VMware balloon driver, the shrinker is disabled by default since we encountered a performance degradation in testing. I tried to avoid rapid inflation/shrinker-deflation cycles by adding a timeout, but apparently it did not help in avoiding the performance regression. So there is no such issue in VMware balloon driver, unless someone intentionally enables the shrinker through a module parameter.
