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=-3.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS autolearn=no 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 49FE6C3A589 for ; Thu, 15 Aug 2019 23:01:00 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id BD63C2064A for ; Thu, 15 Aug 2019 23:00:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="UzD9s477" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BD63C2064A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 43FE06B0005; Thu, 15 Aug 2019 19:00:59 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3F02B6B0006; Thu, 15 Aug 2019 19:00:59 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 3069C6B0007; Thu, 15 Aug 2019 19:00:59 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0020.hostedemail.com [216.40.44.20]) by kanga.kvack.org (Postfix) with ESMTP id 0FC106B0005 for ; Thu, 15 Aug 2019 19:00:59 -0400 (EDT) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id AD7AF1EF3 for ; Thu, 15 Aug 2019 23:00:58 +0000 (UTC) X-FDA: 75826184196.11.power75_8785a30c3e49 X-HE-Tag: power75_8785a30c3e49 X-Filterd-Recvd-Size: 10485 Received: from mail-io1-f67.google.com (mail-io1-f67.google.com [209.85.166.67]) by imf34.hostedemail.com (Postfix) with ESMTP for ; Thu, 15 Aug 2019 23:00:58 +0000 (UTC) Received: by mail-io1-f67.google.com with SMTP id j6so2547372ioa.5 for ; Thu, 15 Aug 2019 16:00:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=kG92dCPdeEwjoRAhcJ0Jn0GMtXx5bG5wbAOXlf/NWg8=; b=UzD9s477YeC4Nx55ghI9V5XO9LG7lxXSqT76wRXdvn936YENf5xjIJgtx6ms7glVq/ eHZuORZVmX9GFUx19mpD+Ln+O8nRP6HVZokbTm3PCRPP6AJyxF9dCDPdvxlcmue6zl4+ 3yvawxyDdU1mZLOIobl5sNHGYz+wBnBD54T8YHmfws57ustZAIhnzcPV42+tuFNlFNFG bB2ytxvc2K3uwVG+rHzaxT04OlceQQqNYlEkC4B6FzRpMn04er1/GWFAgj4KvBRheK+B ksASKviVB4cwfCkgRJhUnHz9R3Y40feal8Aqese4NEec8yuH5Vr5HMBPspI5vBBpei2j LcEg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=kG92dCPdeEwjoRAhcJ0Jn0GMtXx5bG5wbAOXlf/NWg8=; b=SyUR8xTe97GIQSYWHqrR04B6XXxrYvtUrLIvnLT5PBd8r5qkp6CMp/Xj1dkHzm6F2w ah3wD87BdLbSApD2gbOmVo4z7bpR4p2b5SEe8giXNbHmijcsCgIKqx1H1M7llYsnjRcI DgYBBDxibP/ISzSVVrlMn/fNXc64y7a8/mQmgJtvmZeNAcWqDX9m09QAGygnSxih28OX VYeq/SDVnnEc5GzzlGuZsG7lcukPA9q7HBWzobBlqViR2YTy9uiRG1wYo7F328zsXI77 Se3UtNcOUaKiu5i8zdkDA6ETzCgDVWJ2Bdy42RLKkFpCdiBImy3GmaCnx1lhUuVL+6Q5 tGZg== X-Gm-Message-State: APjAAAWvAfJqL/jTiFQcnayHo7OrN+C2fjxTzW4hCaJqMaxtWX8ppk7k A4vtU4sFMsXDb4Nh5mEPi9AyqJYJOP3LYcLmP/I= X-Google-Smtp-Source: APXvYqx9CEr6PuNgFt5Dq9UaLmcsvFhGgGccjwzF2tEUI24gnH2D2zKoCeeKICMbkNLO8k19FSyV1ZWKoCyXAQMiiZM= X-Received: by 2002:a6b:7805:: with SMTP id j5mr8255299iom.42.1565910057260; Thu, 15 Aug 2019 16:00:57 -0700 (PDT) MIME-Version: 1.0 References: <20190812131235.27244-1-nitesh@redhat.com> <20190812131235.27244-2-nitesh@redhat.com> <6d5b57ca-41ff-5c54-ab20-2b1631a6ce29@redhat.com> <09c6fbef-fa53-3a25-d3d6-460b9b6b2020@redhat.com> <6241ef40-9403-1cb0-4e91-a1b86fcf1388@redhat.com> In-Reply-To: <6241ef40-9403-1cb0-4e91-a1b86fcf1388@redhat.com> From: Alexander Duyck Date: Thu, 15 Aug 2019 16:00:46 -0700 Message-ID: Subject: Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure To: Nitesh Narayan Lal Cc: kvm list , LKML , linux-mm , virtio-dev@lists.oasis-open.org, Paolo Bonzini , lcapitulino@redhat.com, Pankaj Gupta , "Wang, Wei W" , Yang Zhang , Rik van Riel , David Hildenbrand , "Michael S. Tsirkin" , dodgen@google.com, Konrad Rzeszutek Wilk , dhildenb@redhat.com, Andrea Arcangeli , john.starks@microsoft.com, Dave Hansen , Michal Hocko , cohuck@redhat.com Content-Type: text/plain; charset="UTF-8" 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 Thu, Aug 15, 2019 at 12:23 PM Nitesh Narayan Lal wrote: > > > On 8/15/19 9:15 AM, Nitesh Narayan Lal wrote: > > On 8/14/19 12:11 PM, Alexander Duyck wrote: > >> On Wed, Aug 14, 2019 at 8:49 AM Nitesh Narayan Lal wrote: > >>> On 8/12/19 2:47 PM, Alexander Duyck wrote: > >>>> On Mon, Aug 12, 2019 at 6:13 AM Nitesh Narayan Lal wrote: > >>>>> This patch introduces the core infrastructure for free page reporting in > >>>>> virtual environments. It enables the kernel to track the free pages which > >>>>> can be reported to its hypervisor so that the hypervisor could > >>>>> free and reuse that memory as per its requirement. > >>>>> > >>>>> While the pages are getting processed in the hypervisor (e.g., > >>>>> via MADV_DONTNEED), the guest must not use them, otherwise, data loss > >>>>> would be possible. To avoid such a situation, these pages are > >>>>> temporarily removed from the buddy. The amount of pages removed > >>>>> temporarily from the buddy is governed by the backend(virtio-balloon > >>>>> in our case). > >>>>> > >>>>> To efficiently identify free pages that can to be reported to the > >>>>> hypervisor, bitmaps in a coarse granularity are used. Only fairly big > >>>>> chunks are reported to the hypervisor - especially, to not break up THP > >>>>> in the hypervisor - "MAX_ORDER - 2" on x86, and to save space. The bits > >>>>> in the bitmap are an indication whether a page *might* be free, not a > >>>>> guarantee. A new hook after buddy merging sets the bits. > >>>>> > >>>>> Bitmaps are stored per zone, protected by the zone lock. A workqueue > >>>>> asynchronously processes the bitmaps, trying to isolate and report pages > >>>>> that are still free. The backend (virtio-balloon) is responsible for > >>>>> reporting these batched pages to the host synchronously. Once reporting/ > >>>>> freeing is complete, isolated pages are returned back to the buddy. > >>>>> > >>>>> Signed-off-by: Nitesh Narayan Lal > >>> [...] > >>>>> +} > >>>>> + > >>>>> +/** > >>>>> + * __page_reporting_enqueue - tracks the freed page in the respective zone's > >>>>> + * bitmap and enqueues a new page reporting job to the workqueue if possible. > >>>>> + */ > >>>>> +void __page_reporting_enqueue(struct page *page) > >>>>> +{ > >>>>> + struct page_reporting_config *phconf; > >>>>> + struct zone *zone; > >>>>> + > >>>>> + rcu_read_lock(); > >>>>> + /* > >>>>> + * We should not process this page if either page reporting is not > >>>>> + * yet completely enabled or it has been disabled by the backend. > >>>>> + */ > >>>>> + phconf = rcu_dereference(page_reporting_conf); > >>>>> + if (!phconf) > >>>>> + return; > >>>>> + > >>>>> + zone = page_zone(page); > >>>>> + bitmap_set_bit(page, zone); > >>>>> + > >>>>> + /* > >>>>> + * We should not enqueue a job if a previously enqueued reporting work > >>>>> + * is in progress or we don't have enough free pages in the zone. > >>>>> + */ > >>>>> + if (atomic_read(&zone->free_pages) >= phconf->max_pages && > >>>>> + !atomic_cmpxchg(&phconf->refcnt, 0, 1)) > >>>> This doesn't make any sense to me. Why are you only incrementing the > >>>> refcount if it is zero? Combining this with the assignment above, this > >>>> isn't really a refcnt. It is just an oversized bitflag. > >>> The intent for having an extra variable was to ensure that at a time only one > >>> reporting job is enqueued. I do agree that for that purpose I really don't need > >>> a reference counter and I should have used something like bool > >>> 'page_hinting_active'. But with bool, I think there could be a possible chance > >>> of race. Maybe I should rename this variable and keep it as atomic. > >>> Any thoughts? > >> You could just use a bitflag to achieve what you are doing here. That > >> is the primary use case for many of the test_and_set_bit type > >> operations. However one issue with doing it as a bitflag is that you > >> have no way of telling that you took care of all requesters. > > I think you are right, I might end up missing on certain reporting > > opportunities in some special cases. Specifically when the pages which are > > part of this new reporting request belongs to a section of the bitmap which > > has already been scanned. Although, I have failed to reproduce this kind of > > situation in an actual setup. > > > >> That is > >> where having an actual reference count comes in handy as you know > >> exactly how many zones are requesting to be reported on. > > > > True. > > > >>>> Also I am pretty sure this results in the opportunity to miss pages > >>>> because there is nothing to prevent you from possibly missing a ton of > >>>> pages you could hint on if a large number of pages are pushed out all > >>>> at once and then the system goes idle in terms of memory allocation > >>>> and freeing. > >>> I was looking at how you are enqueuing/processing reporting jobs for each zone. > >>> I am wondering if I should also consider something on similar lines as having > >>> that I might be able to address the concern which you have raised above. But it > >>> would also mean that I have to add an additional flag in the zone_flags. :) > >> You could do it either in the zone or outside the zone as yet another > >> bitmap. I decided to put the flags inside the zone because there was a > >> number of free bits there and it should be faster since we were > >> already using the zone structure. > > There are two possibilities which could happen while I am reporting: > > 1. Another request might come in for a different zone. > > 2. Another request could come in for the same zone and the pages belong to a > > section of the bitmap which has already been scanned. > > > > Having a per zone flag to indicate reporting status will solve the first > > issue and to an extent the second as well. Having refcnt will possibly solve > > both of them. What I am wondering about is that in my case I could easily > > impact the performance negatively by performing more bitmap scanning. > > > > > > I realized that it may not be possible for me to directly adopt either refcnt > or zone flags just because of the way I have page reporting setup right now. > > For now, I will just replace the refcnt with a bitflag as that should work > for most of the cases. Nevertheless, I will also keep looking for a better way. If nothing else something you could consider is a refcnt for the number of bits you have set in your bitfield. Then all you would need to be doing is replace the cmpxchg with just a atomic_fetch_inc and what you would need to do is have your worker thread track how many bits it has cleared and subtract that from the refcnt at the end.