From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E89E9C10F13 for ; Mon, 8 Apr 2019 18:40:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BCBDA20833 for ; Mon, 8 Apr 2019 18:40:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729152AbfDHSkX (ORCPT ); Mon, 8 Apr 2019 14:40:23 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57736 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726352AbfDHSkX (ORCPT ); Mon, 8 Apr 2019 14:40:23 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 465C6307D91F; Mon, 8 Apr 2019 18:40:22 +0000 (UTC) Received: from [10.36.116.113] (ovpn-116-113.ams2.redhat.com [10.36.116.113]) by smtp.corp.redhat.com (Postfix) with ESMTP id A596F6402B; Mon, 8 Apr 2019 18:40:12 +0000 (UTC) Subject: Re: Thoughts on simple scanner approach for free page hinting To: Alexander Duyck Cc: "Michael S. Tsirkin" , Nitesh Narayan Lal , kvm list , LKML , linux-mm , Paolo Bonzini , lcapitulino@redhat.com, pagupta@redhat.com, wei.w.wang@intel.com, Yang Zhang , Rik van Riel , dodgen@google.com, Konrad Rzeszutek Wilk , dhildenb@redhat.com, Andrea Arcangeli References: <01d5f4e8-742b-33f5-6d91-0c7c396d1cfc@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: Date: Mon, 8 Apr 2019 20:40:11 +0200 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 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Mon, 08 Apr 2019 18:40:22 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> >>> In addition we will need some way to identify which pages have been >>> hinted on and which have not. The way I believe easiest to do this >>> would be to overload the PageType value so that we could essentially >>> have two values for "Buddy" pages. We would have our standard "Buddy" >>> pages, and "Buddy" pages that also have the "Offline" value set in the >>> PageType field. Tracking the Online vs Offline pages this way would >>> actually allow us to do this with almost no overhead as the mapcount >>> value is already being reset to clear the "Buddy" flag so adding a >>> "Offline" flag to this clearing should come at no additional cost. >> >> Just nothing here that this will require modifications to kdump >> (makedumpfile to be precise and the vmcore information exposed from the >> kernel), as kdump only checks for the the actual mapcount value to >> detect buddy and offline pages (to exclude them from dumps), they are >> not treated as flags. >> >> For now, any mapcount values are really only separate values, meaning >> not the separate bits are of interest, like flags would be. Reusing >> other flags would make our life a lot easier. E.g. PG_young or so. But >> clearing of these is then the problematic part. >> >> Of course we could use in the kernel two values, Buddy and BuddyOffline. >> But then we have to check for two different values whenever we want to >> identify a buddy page in the kernel. > > Actually this may not be working the way you think it is working. Trust me, I know how it works. That's why I was giving you the notice. Read the first paragraph again and ignore the others. I am only concerned about makedumpfile that has to be changed. PAGE_OFFLINE_MAPCOUNT_VALUE PAGE_BUDDY_MAPCOUNT_VALUE Once you find out how these values are used, you should understand what has to be changed and where. >>> >>> Lastly we would need to create a specialized function for allocating >>> the non-"Offline" pages, and to tweak __free_one_page to tail enqueue >>> "Offline" pages. I'm thinking the alloc function it would look >>> something like __rmqueue_smallest but without the "expand" and needing >>> to modify the !page check to also include a check to verify the page >>> is not "Offline". As far as the changes to __free_one_page it would be >>> a 2 line change to test for the PageType being offline, and if it is >>> to call add_to_free_area_tail instead of add_to_free_area. >> >> As already mentioned, there might be scenarios where the additional >> hinting thread might consume too much CPU cycles, especially if there is >> little guest activity any you mostly spend time scanning a handful of >> free pages and reporting them. I wonder if we can somehow limit the >> amount of wakeups/scans for a given period to mitigate this issue. > > That is why I was talking about breaking nr_free into nr_freed and > nr_bound. By doing that I can record the nr_free value to a > virtio-balloon specific location at the start of any walk and should > know exactly now many pages were freed between that call and the next > one. By ordering things such that we place the "Offline" pages on the > tail of the list it should make the search quite fast since we would > just be always allocating off of the head of the queue until we have > hinted everything int he queue. So when we hit the last call to alloc > the non-"Offline" pages and shut down our thread we can use the > nr_freed value that we recorded to know exactly how many pages have > been added that haven't been hinted. > >> One main issue I see with your approach is that we need quite a lot of >> core memory management changes. This is a problem. I wonder if we can >> factor out most parts into callbacks. > > I think that is something we can't get away from. However if we make > this generic enough there would likely be others beyond just the > virtualization drivers that could make use of the infrastructure. For > example being able to track the rate at which the free areas are > cycling in and out pages seems like something that would be useful > outside of just the virtualization areas. Might be, but might be the other extreme, people not wanting such special cases in core mm. I assume the latter until I see a very clear design where such stuff has been properly factored out. > >> E.g. in order to detect where to queue a certain page (front/tail), call >> a callback if one is registered, mark/check pages in a core-mm unknown >> way as offline etc. >> >> I still wonder if there could be an easier way to combine recording of >> hints and one hinting thread, essentially avoiding scanning and some of >> the required core-mm changes. > > The concern I have with trying to avoid the scanning by tracking is > that if you fall behind it becomes something where just tracking the > metadata for the page hints would start to become expensive. Depends, if it is mostly only marking a bit in a bitmap, it should in general not be too much of an issue. As usual, the datastructure used is the important bit. -- Thanks, David / dhildenb