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Tsirkin" Cc: Nadav Amit , Alexander Duyck , Tyler Sanderson , "Wang, Wei W" , "virtualization@lists.linux-foundation.org" , David Rientjes , "linux-mm@kvack.org" , Michal Hocko References: <75d4594f-0864-5172-a0f8-f97affedb366@redhat.com> <286AC319A985734F985F78AFA26841F73E3F8A02@shsmsx102.ccr.corp.intel.com> <20200203080520-mutt-send-email-mst@kernel.org> <5ac131de8e3b7fc1fafd05a61feb5f6889aeb917.camel@linux.intel.com> <539B606A-A0CA-4AA4-B99A-759C874A1EF8@vmware.com> <08D323B9-0B4A-47B2-9955-511B8FBA056B@vmware.com> <20200205052548-mutt-send-email-mst@kernel.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= mQINBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMFCQlmAYAGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheAFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl3pImkCGQEACgkQTd4Q 9wD/g1o+VA//SFvIHUAvul05u6wKv/pIR6aICPdpF9EIgEU448g+7FfDgQwcEny1pbEzAmiw zAXIQ9H0NZh96lcq+yDLtONnXk/bEYWHHUA014A1wqcYNRY8RvY1+eVHb0uu0KYQoXkzvu+s Dncuguk470XPnscL27hs8PgOP6QjG4jt75K2LfZ0eAqTOUCZTJxA8A7E9+XTYuU0hs7QVrWJ jQdFxQbRMrYz7uP8KmTK9/Cnvqehgl4EzyRaZppshruKMeyheBgvgJd5On1wWq4ZUV5PFM4x II3QbD3EJfWbaJMR55jI9dMFa+vK7MFz3rhWOkEx/QR959lfdRSTXdxs8V3zDvChcmRVGN8U Vo93d1YNtWnA9w6oCW1dnDZ4kgQZZSBIjp6iHcA08apzh7DPi08jL7M9UQByeYGr8KuR4i6e RZI6xhlZerUScVzn35ONwOC91VdYiQgjemiVLq1WDDZ3B7DIzUZ4RQTOaIWdtXBWb8zWakt/ ztGhsx0e39Gvt3391O1PgcA7ilhvqrBPemJrlb9xSPPRbaNAW39P8ws/UJnzSJqnHMVxbRZC Am4add/SM+OCP0w3xYss1jy9T+XdZa0lhUvJfLy7tNcjVG/sxkBXOaSC24MFPuwnoC9WvCVQ ZBxouph3kqc4Dt5X1EeXVLeba+466P1fe1rC8MbcwDkoUo65Ag0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAGJAiUEGAECAA8FAlXLn5ECGwwFCQlmAYAACgkQTd4Q 9wD/g1qA6w/+M+ggFv+JdVsz5+ZIc6MSyGUozASX+bmIuPeIecc9UsFRatc91LuJCKMkD9Uv GOcWSeFpLrSGRQ1Z7EMzFVU//qVs6uzhsNk0RYMyS0B6oloW3FpyQ+zOVylFWQCzoyyf227y GW8HnXunJSC+4PtlL2AY4yZjAVAPLK2l6mhgClVXTQ/S7cBoTQKP+jvVJOoYkpnFxWE9pn4t H5QIFk7Ip8TKr5k3fXVWk4lnUi9MTF/5L/mWqdyIO1s7cjharQCstfWCzWrVeVctpVoDfJWp 4LwTuQ5yEM2KcPeElLg5fR7WB2zH97oI6/Ko2DlovmfQqXh9xWozQt0iGy5tWzh6I0JrlcxJ ileZWLccC4XKD1037Hy2FLAjzfoWgwBLA6ULu0exOOdIa58H4PsXtkFPrUF980EEibUp0zFz GotRVekFAceUaRvAj7dh76cToeZkfsjAvBVb4COXuhgX6N4pofgNkW2AtgYu1nUsPAo+NftU CxrhjHtLn4QEBpkbErnXQyMjHpIatlYGutVMS91XTQXYydCh5crMPs7hYVsvnmGHIaB9ZMfB njnuI31KBiLUks+paRkHQlFcgS2N3gkRBzH7xSZ+t7Re3jvXdXEzKBbQ+dC3lpJB0wPnyMcX FOTT3aZT7IgePkt5iC/BKBk3hqKteTnJFeVIT7EC+a6YUFg= Organization: Red Hat GmbH Message-ID: Date: Wed, 5 Feb 2020 11:43:49 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20200205052548-mutt-send-email-mst@kernel.org> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-MC-Unique: DTQQek8TOJm5LGjUJBSIFA-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 wrote: >>>> >>>>> On Feb 3, 2020, at 8:34 AM, David Hildenbrand wrot= e: >>>>> >>>>> 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 = 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 >>>>>>>>> > 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 >>>>>>>>> >>>>>>>> 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 memor= y >>>>>>>>> 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=E2=80=99t invok= e 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 star= ts 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 notifi= er with shrinker") >>>>>>> Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT >>>>>>> shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't n= ice >>>>>>> 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 mu= ch >>>>>> value to having a shrinker running deflation when you are actively t= rying >>>>>> 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 alway= s >>>>> 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 OO= M >>>>> notifier) >>>> >>>> David, >>>> >>>> Please keep me posted. I decided to adapt the same solution as the vir= tio >>>> balloon for the VMware balloon. If the verdict is that this is damagin= g 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 rapi= d >>> 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". >=20 > So as you pointed out yourself deflate on oom is really under-specified. >=20 > 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). --=20 Thanks, David / dhildenb