Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page
 reporting
To:     David Hildenbrand <david@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Rik van Riel <riel@surriel.com>, lcapitulino@redhat.com,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>
References: <20191001152441.27008.99285.stgit@localhost.localdomain>
 <7233498c-2f64-d661-4981-707b59c78fd5@redhat.com>
 <1ea1a4e11617291062db81f65745b9c95fd0bb30.camel@linux.intel.com>
 <8bd303a6-6e50-b2dc-19ab-4c3f176c4b02@redhat.com>
 <CAKgT0Uf37xAFK2CWqUZJgn7bWznSAi6qncLxBpC55oSpBMG1HQ@mail.gmail.com>
 <0603ca4a-d667-f461-4ba7-ff3c0e9fd4df@redhat.com>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
Openpgp: preference=signencrypt
Autocrypt: addr=nitesh@redhat.com; prefer-encrypt=mutual; keydata=
 mQINBFl4pQoBEADT/nXR2JOfsCjDgYmE2qonSGjkM1g8S6p9UWD+bf7YEAYYYzZsLtbilFTe
 z4nL4AV6VJmC7dBIlTi3Mj2eymD/2dkKP6UXlliWkq67feVg1KG+4UIp89lFW7v5Y8Muw3Fm
 uQbFvxyhN8n3tmhRe+ScWsndSBDxYOZgkbCSIfNPdZrHcnOLfA7xMJZeRCjqUpwhIjxQdFA7
 n0s0KZ2cHIsemtBM8b2WXSQG9CjqAJHVkDhrBWKThDRF7k80oiJdEQlTEiVhaEDURXq+2XmG
 jpCnvRQDb28EJSsQlNEAzwzHMeplddfB0vCg9fRk/kOBMDBtGsTvNT9OYUZD+7jaf0gvBvBB
 lbKmmMMX7uJB+ejY7bnw6ePNrVPErWyfHzR5WYrIFUtgoR3LigKnw5apzc7UIV9G8uiIcZEn
 C+QJCK43jgnkPcSmwVPztcrkbC84g1K5v2Dxh9amXKLBA1/i+CAY8JWMTepsFohIFMXNLj+B
 RJoOcR4HGYXZ6CAJa3Glu3mCmYqHTOKwezJTAvmsCLd3W7WxOGF8BbBjVaPjcZfavOvkin0u
 DaFvhAmrzN6lL0msY17JCZo046z8oAqkyvEflFbC0S1R/POzehKrzQ1RFRD3/YzzlhmIowkM
 BpTqNBeHEzQAlIhQuyu1ugmQtfsYYq6FPmWMRfFPes/4JUU/PQARAQABtCVOaXRlc2ggTmFy
 YXlhbiBMYWwgPG5pbGFsQHJlZGhhdC5jb20+iQI9BBMBCAAnBQJZeKUKAhsjBQkJZgGABQsJ
 CAcCBhUICQoLAgQWAgMBAh4BAheAAAoJEKOGQNwGMqM56lEP/A2KMs/pu0URcVk/kqVwcBhU
 SnvB8DP3lDWDnmVrAkFEOnPX7GTbactQ41wF/xwjwmEmTzLrMRZpkqz2y9mV0hWHjqoXbOCS
 6RwK3ri5e2ThIPoGxFLt6TrMHgCRwm8YuOSJ97o+uohCTN8pmQ86KMUrDNwMqRkeTRW9wWIQ
 EdDqW44VwelnyPwcmWHBNNb1Kd8j3xKlHtnS45vc6WuoKxYRBTQOwI/5uFpDZtZ1a5kq9Ak/
 MOPDDZpd84rqd+IvgMw5z4a5QlkvOTpScD21G3gjmtTEtyfahltyDK/5i8IaQC3YiXJCrqxE
 r7/4JMZeOYiKpE9iZMtS90t4wBgbVTqAGH1nE/ifZVAUcCtycD0f3egX9CHe45Ad4fsF3edQ
 ESa5tZAogiA4Hc/yQpnnf43a3aQ67XPOJXxS0Qptzu4vfF9h7kTKYWSrVesOU3QKYbjEAf95
 NewF9FhAlYqYrwIwnuAZ8TdXVDYt7Z3z506//sf6zoRwYIDA8RDqFGRuPMXUsoUnf/KKPrtR
 ceLcSUP/JCNiYbf1/QtW8S6Ca/4qJFXQHp0knqJPGmwuFHsarSdpvZQ9qpxD3FnuPyo64S2N
 Dfq8TAeifNp2pAmPY2PAHQ3nOmKgMG8Gn5QiORvMUGzSz8Lo31LW58NdBKbh6bci5+t/HE0H
 pnyVf5xhNC/FuQINBFl4pQoBEACr+MgxWHUP76oNNYjRiNDhaIVtnPRqxiZ9v4H5FPxJy9UD
 Bqr54rifr1E+K+yYNPt/Po43vVL2cAyfyI/LVLlhiY4yH6T1n+Di/hSkkviCaf13gczuvgz4
 KVYLwojU8+naJUsiCJw01MjO3pg9GQ+47HgsnRjCdNmmHiUQqksMIfd8k3reO9SUNlEmDDNB
 XuSzkHjE5y/R/6p8uXaVpiKPfHoULjNRWaFc3d2JGmxJpBdpYnajoz61m7XJlgwl/B5Ql/6B
 dHGaX3VHxOZsfRfugwYF9CkrPbyO5PK7yJ5vaiWre7aQ9bmCtXAomvF1q3/qRwZp77k6i9R3
 tWfXjZDOQokw0u6d6DYJ0Vkfcwheg2i/Mf/epQl7Pf846G3PgSnyVK6cRwerBl5a68w7xqVU
 4KgAh0DePjtDcbcXsKRT9D63cfyfrNE+ea4i0SVik6+N4nAj1HbzWHTk2KIxTsJXypibOKFX
 2VykltxutR1sUfZBYMkfU4PogE7NjVEU7KtuCOSAkYzIWrZNEQrxYkxHLJsWruhSYNRsqVBy
 KvY6JAsq/i5yhVd5JKKU8wIOgSwC9P6mXYRgwPyfg15GZpnw+Fpey4bCDkT5fMOaCcS+vSU1
 UaFmC4Ogzpe2BW2DOaPU5Ik99zUFNn6cRmOOXArrryjFlLT5oSOe4IposgWzdwARAQABiQIl
 BBgBCAAPBQJZeKUKAhsMBQkJZgGAAAoJEKOGQNwGMqM5ELoP/jj9d9gF1Al4+9bngUlYohYu
 0sxyZo9IZ7Yb7cHuJzOMqfgoP4tydP4QCuyd9Q2OHHL5AL4VFNb8SvqAxxYSPuDJTI3JZwI7
 d8JTPKwpulMSUaJE8ZH9n8A/+sdC3CAD4QafVBcCcbFe1jifHmQRdDrvHV9Es14QVAOTZhnJ
 vweENyHEIxkpLsyUUDuVypIo6y/Cws+EBCWt27BJi9GH/EOTB0wb+2ghCs/i3h8a+bi+bS7L
 FCCm/AxIqxRurh2UySn0P/2+2eZvneJ1/uTgfxnjeSlwQJ1BWzMAdAHQO1/lnbyZgEZEtUZJ
 x9d9ASekTtJjBMKJXAw7GbB2dAA/QmbA+Q+Xuamzm/1imigz6L6sOt2n/X/SSc33w8RJUyor
 SvAIoG/zU2Y76pKTgbpQqMDmkmNYFMLcAukpvC4ki3Sf086TdMgkjqtnpTkEElMSFJC8npXv
 3QnGGOIfFug/qs8z03DLPBz9VYS26jiiN7QIJVpeeEdN/LKnaz5LO+h5kNAyj44qdF2T2AiF
 HxnZnxO5JNP5uISQH3FjxxGxJkdJ8jKzZV7aT37sC+Rp0o3KNc+GXTR+GSVq87Xfuhx0LRST
 NK9ZhT0+qkiN7npFLtNtbzwqaqceq3XhafmCiw8xrtzCnlB/C4SiBr/93Ip4kihXJ0EuHSLn
 VujM7c/b4pps
Organization: Red Hat Inc,
Message-ID: <020ef7af-dae7-4d57-bed8-1ce912d50c1d@redhat.com>
Date:   Wed, 2 Oct 2019 06:44:48 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <0603ca4a-d667-f461-4ba7-ff3c0e9fd4df@redhat.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8BIT
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 10/2/19 3:13 AM, David Hildenbrand wrote:
> On 02.10.19 02:55, Alexander Duyck wrote:
>> On Tue, Oct 1, 2019 at 12:16 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>
>>> On 10/1/19 12:21 PM, Alexander Duyck wrote:
>>>> On Tue, 2019-10-01 at 17:35 +0200, David Hildenbrand wrote:
>>>>> On 01.10.19 17:29, Alexander Duyck wrote:
>>>>>> This series provides an asynchronous means of reporting to a hypervisor
>>>>>> that a guest page is no longer in use and can have the data associated
>>>>>> with it dropped. To do this I have implemented functionality that allows
>>>>>> for what I am referring to as unused page reporting. The advantage of
>>>>>> unused page reporting is that we can support a significant amount of
>>>>>> memory over-commit with improved performance as we can avoid having to
>>>>>> write/read memory from swap as the VM will instead actively participate
>>>>>> in freeing unused memory so it doesn't have to be written.
>>>>>>
>>>>>> The functionality for this is fairly simple. When enabled it will allocate
>>>>>> statistics to track the number of reported pages in a given free area.
>>>>>> When the number of free pages exceeds this value plus a high water value,
>>>>>> currently 32, it will begin performing page reporting which consists of
>>>>>> pulling non-reported pages off of the free lists of a given zone and
>>>>>> placing them into a scatterlist. The scatterlist is then given to the page
>>>>>> reporting device and it will perform the required action to make the pages
>>>>>> "reported", in the case of virtio-balloon this results in the pages being
>>>>>> madvised as MADV_DONTNEED. After this they are placed back on their
>>>>>> original free list. If they are not merged in freeing an additional bit is
>>>>>> set indicating that they are a "reported" buddy page instead of a standard
>>>>>> buddy page. The cycle then repeats with additional non-reported pages
>>>>>> being pulled until the free areas all consist of reported pages.
>>>>>>
>>>>>> In order to try and keep the time needed to find a non-reported page to
>>>>>> a minimum we maintain a "reported_boundary" pointer. This pointer is used
>>>>>> by the get_unreported_pages iterator to determine at what point it should
>>>>>> resume searching for non-reported pages. In order to guarantee pages do
>>>>>> not get past the scan I have modified add_to_free_list_tail so that it
>>>>>> will not insert pages behind the reported_boundary. Doing this allows us
>>>>>> to keep the overhead to a minimum as re-walking the list without the
>>>>>> boundary will result in as much as 18% additional overhead on a 32G VM.
>>>>>>
>>>>>>
>>>> <snip>
>>>>
>>>>>> As far as possible regressions I have focused on cases where performing
>>>>>> the hinting would be non-optimal, such as cases where the code isn't
>>>>>> needed as memory is not over-committed, or the functionality is not in
>>>>>> use. I have been using the will-it-scale/page_fault1 test running with 16
>>>>>> vcpus and have modified it to use Transparent Huge Pages. With this I see
>>>>>> almost no difference with the patches applied and the feature disabled.
>>>>>> Likewise I see almost no difference with the feature enabled, but the
>>>>>> madvise disabled in the hypervisor due to a device being assigned. With
>>>>>> the feature fully enabled in both guest and hypervisor I see a regression
>>>>>> between -1.86% and -8.84% versus the baseline. I found that most of the
>>>>>> overhead was due to the page faulting/zeroing that comes as a result of
>>>>>> the pages having been evicted from the guest.
>>>>> I think Michal asked for a performance comparison against Nitesh's
>>>>> approach, to evaluate if keeping the reported state + tracking inside
>>>>> the buddy is really worth it. Do you have any such numbers already? (or
>>>>> did my tired eyes miss them in this cover letter? :/)
>>>>>
>>>> I thought what Michal was asking for was what was the benefit of using the
>>>> boundary pointer. I added a bit up above and to the description for patch
>>>> 3 as on a 32G VM it adds up to about a 18% difference without factoring in
>>>> the page faulting and zeroing logic that occurs when we actually do the
>>>> madvise.
>>>>
>>>> Do we have a working patch set for Nitesh's code? The last time I tried
>>>> running his patch set I ran into issues with kernel panics. If we have a
>>>> known working/stable patch set I can give it a try.
>>> Did you try the v12 patch-set [1]?
>>> I remember that you reported the CPU stall issue, which I fixed in the v12.
>>>
>>> [1] https://lkml.org/lkml/2019/8/12/593
>> So I tried testing with the spin_lock calls replaced with spin_lock
>> _irq to resolve the IRQ issue. I also had shuffle enabled in order to
>> increase the number of pages being dirtied.
>>
>> With that setup the bitmap approach is running significantly worse
>> then my approach, even with the boundary removed. Since I had to
> It would make sense to share the setup+benchmark+performance indication
> that you measured. You don't have to share the actual numbers.

+1

>
>> modify the code to even getting working I am not comfortable posting
>> numbers. My suggestion would be to look at reworking the patch set and
>> post numbers for my patch set versus the bitmap approach and we can
>> look at them then. I would prefer not to spend my time fixing and
>> tuning a patch set that I am still not convinced is viable.
> I agree, I think Nitesh should work on his patch set and try to
> reproduce what you are seeing.

Sure.
I am always open to suggestions of different benchmarks/setup
where I can run my patch-set.

>
> Also, I think to make a precise statement of "which overhead comes with
> external tracking", Nitesh should switch to an approach (motivated by
> Michal) like
>
> 1. Sense lockless if a page is still free
> 2. start_isolate_page_range()
> -> Failed? Skip
> 3. test_pages_isolated()
> -> No? undo_isolate_page_range(), skip
> 4. Repeat for multiple pages + report
> 5. undo_isolate_page_range()
>
> That is the bare minimum any external tracking will need = some overhead
> for the tracking data. As a nice side effect, it get's rid of taking the
> zone lock manually AFAIKS.
>
> But that's unrelated to your series, only to quantify "how much" does
> external tracking actually cost.

Exactly, first, we need to be sure that the overhead caused by bitmap
scanning is not significant. If we are fine with the approach, I will
certainly look into this as this would be an excellent enhancement.

-- 
Thanks
Nitesh