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[209.132.183.28]) by mx.google.com with ESMTPS id m4si10178135vsd.38.2019.07.25.11.26.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Jul 2019 11:26:05 -0700 (PDT) Received-SPF: pass (google.com: domain of nitesh@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of nitesh@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=nitesh@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 2C26D30001D8; Thu, 25 Jul 2019 18:26:04 +0000 (UTC) Received: from [10.18.17.163] (dhcp-17-163.bos.redhat.com [10.18.17.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CD0165E1B0; Thu, 25 Jul 2019 18:25:53 +0000 (UTC) Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting" To: Alexander Duyck , "Michael S. Tsirkin" Cc: Alexander Duyck , kvm@vger.kernel.org, david@redhat.com, dave.hansen@intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, yang.zhang.wz@gmail.com, pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com, lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com References: <20190724165158.6685.87228.stgit@localhost.localdomain> <20190724171050.7888.62199.stgit@localhost.localdomain> <20190724173403-mutt-send-email-mst@kernel.org> <20190725111303-mutt-send-email-mst@kernel.org> <96b1ac42dccbfbb5dd17210e6767ca2544558390.camel@linux.intel.com> From: Nitesh Narayan Lal 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: Date: Thu, 25 Jul 2019 14:25:52 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <96b1ac42dccbfbb5dd17210e6767ca2544558390.camel@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.46]); Thu, 25 Jul 2019 18:26:04 +0000 (UTC) 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 7/25/19 12:16 PM, Alexander Duyck wrote: > On Thu, 2019-07-25 at 11:16 -0400, Michael S. Tsirkin wrote: >> On Thu, Jul 25, 2019 at 08:05:30AM -0700, Alexander Duyck wrote: >>> On Thu, 2019-07-25 at 07:35 -0400, Nitesh Narayan Lal wrote: >>>> On 7/24/19 6:03 PM, Alexander Duyck wrote: >>>>> On Wed, 2019-07-24 at 17:38 -0400, Michael S. Tsirkin wrote: >>>>>> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote: >>>>>>> From: Alexander Duyck >>>>>>> >>>>>>> Add support for what I am referring to as "bubble hinting". Basic= ally the >>>>>>> idea is to function very similar to how the balloon works in that= we >>>>>>> basically end up madvising the page as not being used. However we= don't >>>>>>> really need to bother with any deflate type logic since the page = will be >>>>>>> faulted back into the guest when it is read or written to. >>>>>>> >>>>>>> This is meant to be a simplification of the existing balloon inte= rface >>>>>>> to use for providing hints to what memory needs to be freed. I am= assuming >>>>>>> this is safe to do as the deflate logic does not actually appear = to do very >>>>>>> much other than tracking what subpages have been released and whi= ch ones >>>>>>> haven't. >>>>>>> >>>>>>> Signed-off-by: Alexander Duyck >>>>>> BTW I wonder about migration here. When we migrate we lose all hi= nts >>>>>> right? Well destination could be smarter, detect that page is ful= l of >>>>>> 0s and just map a zero page. Then we don't need a hint as such - b= ut I >>>>>> don't think it's done like that ATM. >>>>> I was wondering about that a bit myself. If you migrate with a ball= oon >>>>> active what currently happens with the pages in the balloon? Do you= >>>>> actually migrate them, or do you ignore them and just assume a zero= page? >>>>> I'm just reusing the ram_block_discard_range logic that was being u= sed for >>>>> the balloon inflation so I would assume the behavior would be the s= ame. >>>> I agree, however, I think it is worth investigating to see if enabli= ng hinting >>>> adds some sort of overhead specifically in this kind of scenarios. W= hat do you >>>> think? >>> I suspect that the hinting/reporting would probably improve migration= >>> times based on the fact that from the sound of things it would just b= e >>> migrated as a zero page. >>> >>> I don't have a good setup for testing migration though and I am not t= hat >>> familiar with trying to do a live migration. That is one of the reaso= ns >>> why I didn't want to stray too far from the existing balloon code as = that >>> has already been tested with migration so I would assume as long as I= am >>> doing almost the exact same thing to hint the pages away it should be= have >>> exactly the same. >>> >>>>>> I also wonder about interaction with deflate. ATM deflate will ad= d >>>>>> pages to the free list, then balloon will come right back and repo= rt >>>>>> them as free. >>>>> I don't know how likely it is that somebody who is getting the free= page >>>>> reporting is likely to want to also use the balloon to take up memo= ry. >>>> I think it is possible. There are two possibilities: >>>> 1. User has a workload running, which is allocating and freeing the = pages and at >>>> the same time, user deflates. >>>> If these new pages get used by this workload, we don't have to worry= as you are >>>> already handling that by not hinting the free pages immediately. >>>> 2. Guest is idle and the user adds up some memory, for this situatio= n what you >>>> have explained below does seems reasonable. >>> Us hinting on pages that are freed up via deflate wouldn't be too big= of a >>> deal. I would think that is something we could look at addressing as = more >>> of a follow-on if we ever needed to since it would just add more >>> complexity. >>> >>> Really what I would like to see is the balloon itself get updated fir= st to >>> perhaps work with variable sized pages first so that we could then ha= ve >>> pages come directly out of the balloon and go back into the freelist = as >>> hinted, or visa-versa where hinted pages could be pulled directly int= o the >>> balloon without needing to notify the host. >> Right, I agree. At this point the main thing I worry about is that >> the interfaces only support one reporter, since a page flag is used. >> So if we ever rewrite existing hinting to use the new mm >> infrastructure then we can't e.g. enable both types of hinting. > Does it make sense to have multiple types of hinting active at the same= > time though? That kind of seems wasteful to me.=20 I agree. > Ideally we should be able > to provide the hints and have them feed whatever is supposed to be usin= g > them. So for example I could probably look at also clearing the bitmaps= > when migration is in process. > > Also, I am wonder if the free page hints would be redundant with the fo= rm > of page hinting/reporting that I have since we should be migrating a mu= ch > smaller footprint anyway if the pages have been madvised away before we= > even start the migration. > >> FWIW Nitesh's RFC does not have this limitation. > Yes, but there are also limitations to his approach. For example the fa= ct > that the bitmap it maintains is back to being a hint rather then being > very exact. True. > As a result you could end up walking the bitmap for a while > clearing bits without ever finding a free page. Are referring to the overhead which will be introduced due to bitmap scan= ning on very large guests? > >> I intend to think about this over the weekend. > Sounds good. I'll try to get the stuff you have pointed out so far > addressed and hopefully have v3 ready to go next week. > > Thanks. > > - Alex > --=20 Thanks Nitesh