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Mon, 18 Feb 2019 16:03:05 +0000 (UTC) Received: from [10.36.116.84] (ovpn-116-84.ams2.redhat.com [10.36.116.84]) by smtp.corp.redhat.com (Postfix) with ESMTP id A0A9D5C1B2; Mon, 18 Feb 2019 16:02:52 +0000 (UTC) Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting To: Nitesh Narayan Lal , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, lcapitulino@redhat.com, pagupta@redhat.com, wei.w.wang@intel.com, yang.zhang.wz@gmail.com, riel@surriel.com, mst@redhat.com, dodgen@google.com, konrad.wilk@oracle.com, dhildenb@redhat.com, aarcange@redhat.com, Alexander Duyck References: <20190204201854.2328-1-nitesh@redhat.com> From: David Hildenbrand Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFXLn5EBEAC+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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+ xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl SE+xAvmumFBY Organization: Red Hat GmbH Message-ID: <17dcd165-10c2-2153-2914-e610d8e053ea@redhat.com> Date: Mon, 18 Feb 2019 17:02:51 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit 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 [10.5.110.27]); Mon, 18 Feb 2019 16:03:06 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18.02.19 16:50, Nitesh Narayan Lal wrote: > > On 2/16/19 4:40 AM, David Hildenbrand wrote: >> On 04.02.19 21:18, Nitesh Narayan Lal wrote: >> >> Hi Nitesh, >> >> I thought again about how s390x handles free page hinting. As that seems >> to work just fine, I guess sticking to a similar model makes sense. >> >> >> I already explained in this thread how it works on s390x, a short summary: >> >> 1. Each VCPU has a buffer of pfns to be reported to the hypervisor. If I >> am not wrong, it contains 512 entries, so is exactly 1 page big. This >> buffer is stored in the hypervisor and is on page granularity. >> >> 2. This page buffer is managed via the ESSA instruction. In addition, to >> synchronize with the guest ("page reused when freeing in the >> hypervisor"), special bits in the host->guest page table can be >> set/locked via the ESSA instruction by the guest and similarly accessed >> by the hypervisor. >> >> 3. Once the buffer is full, the guest does a synchronous hypercall, >> going over all 512 entries and zapping them (== similar to MADV_DONTNEED) >> >> >> To mimic that, we >> >> 1. Have a static buffer per VCPU in the guest with 512 entries. You >> basically have that already. >> >> 2. On every free, add the page _or_ the page after merging by the buddy >> (e.g. MAX_ORDER - 1) to the buffer (this is where we could be better >> than s390x). You basically have that already. >> >> 3. If the buffer is full, try to isolate all pages and do a synchronous >> report to the hypervisor. You have the first part already. The second >> part would require a change (don't use a separate/global thread to do >> the hinting, just do it synchronously). >> >> 4. One hinting is done, putback all isolated pages to the budy. You >> basically have that already. >> >> >> For 3. we can try what you have right now, using virtio. If we detect >> that's a problem, we can do it similar to what Alexander proposes and >> just do a bare hypercall. It's just a different way of carrying out the >> same task. >> >> >> This approach >> 1. Mimics what s390x does, besides supporting different granularities. >> To synchronize guest->host we simply take the pages off the buddy. >> >> 2. Is basically what Alexander does, however his design limitation is >> that doing any hinting on smaller granularities will not work because >> there will be too many synchronous hints. Bad on fragmented guests. >> >> 3. Does not require any dynamic data structures in the guest. >> >> 4. Does not block allocation paths. >> >> 5. Blocks on e.g. every 512'ed free. It seems to work on s390x, why >> shouldn't it for us. We have to measure. >> >> 6. We are free to decide which granularity we report. >> >> 7. Potentially works even if the guest memory is fragmented (little >> MAX_ORDER - 1) pages. >> >> It would be worth a try. My feeling is that a synchronous report after >> e.g. 512 frees should be acceptable, as it seems to be acceptable on >> s390x. (basically always enabled, nobody complains). > > The reason I like the current approach of reporting via separate kernel > thread is that it doesn't block any regular allocation/freeing code path > in anyways. Well, that is partially true. The work has to be done "somewhere", so once you kick a separate kernel thread, it can easily be scheduled on the very same VCPU in the very near future. So depending on the user, the "hickup" is similarly visible. Having separate kernel threads seems to result in other questions not easy to answer (do we need dynamic data structures, how to size these data structures, how many threads do we want (e.g. big number of vcpus) ) that seem to be avoidable by keeping it simple and not having separate threads. Initially I also thought that separate threads were the natural thing to do, but now I have the feeling that it tends to over complicate the problem. (and I don't want to repeat myself, but on s390x it seems to work this way just fine, if we want to mimic that). Especially without us knowing if "don't do a hypercall every X free calls" is really a problem. >> >> We would have to play with how to enable/disable reporting and when to >> not report because it's not worth it in the guest (e.g. low on memory). >> >> >> Do you think something like this would be easy to change/implement and >> measure? > > I can do that as I figure out a real world guest workload using which > the two approaches can be compared. -- Thanks, David / dhildenb