Re: Balloon pressuring page cache

From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: Nadav Amit <namit@vmware.com>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Tyler Sanderson <tysand@google.com>,
	"Wang, Wei W" <wei.w.wang@intel.com>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Michal Hocko <mhocko@kernel.org>
Subject: Re: Balloon pressuring page cache
Date: Wed, 5 Feb 2020 05:27:44 -0500	[thread overview]
Message-ID: <20200205052548-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <dd5aa189-daed-7008-d02b-4e5bd61fe6e1@redhat.com>

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@vmware.com> wrote:
> >>
> >>> On Feb 3, 2020, at 8:34 AM, David Hildenbrand <david@redhat.com> 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@intel.com> 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@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/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.

> 
> -- 
> Thanks,
> 
> David / dhildenb