On 05.02.20 11:27, Michael S. Tsirkin wrote: > On Wed, Feb 05, 2020 at 09:19:58AM +0100, David Hildenbrand wrote: >> On 05.02.20 08:35, Nadav Amit wrote: >>>> 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. >> >> Thanks for that info. To me that sounds like the shrinker is the wrong >> approach to "auto-deflation". It's not just "some slab cache". > > So as you pointed out yourself deflate on oom is really under-specified. > > I would be very happy if you could take a stub at documenting what's > expected from guest and how it could be used. > Please copy the virtio TC when you do this as this is spec stuff. So, I'll first get the code back into the desired state, so at least we have an agreement of how it should be, and then follow up with a spec update. Might take some time, though (plenty of stuff to do). -- Thanks, David / dhildenb
