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Thu, 22 Aug 2019 16:19:13 +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 782085D784; Thu, 22 Aug 2019 16:18:57 +0000 (UTC) Subject: Re: [virtio-dev] [PATCH v6 4/6] mm: Introduce Reported pages To: Alexander Duyck , kvm@vger.kernel.org, mst@redhat.com, david@redhat.com, dave.hansen@intel.com, linux-kernel@vger.kernel.org, willy@infradead.org, mhocko@kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, virtio-dev@lists.oasis-open.org, osalvador@suse.de Cc: 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, alexander.h.duyck@linux.intel.com References: <20190821145806.20926.22448.stgit@localhost.localdomain> <20190821145950.20926.83684.stgit@localhost.localdomain> From: Nitesh Narayan Lal Openpgp: preference=signencrypt Autocrypt: addr=nitesh@redhat.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190821145950.20926.83684.stgit@localhost.localdomain> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Thu, 22 Aug 2019 16:19:13 +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 8/21/19 10:59 AM, Alexander Duyck wrote: > From: Alexander Duyck > > In order to pave the way for free page reporting in virtualized > environments we will need a way to get pages out of the free lists and > identify those pages after they have been returned. To accomplish this,= > this patch adds the concept of a Reported Buddy, which is essentially > meant to just be the Uptodate flag used in conjunction with the Buddy > page type. > > It adds a set of pointers we shall call "boundary" which represents the= > upper boundary between the unreported and reported pages. The general i= dea > is that in order for a page to cross from one side of the boundary to t= he > other it will need to go through the reporting process. Ultimately a > free_list has been fully processed when the boundary has been moved fro= m > the tail all they way up to occupying the first entry in the list. > > Doing this we should be able to make certain that we keep the reported > pages as one contiguous block in each free list. This will allow us to > efficiently manipulate the free lists whenever we need to go in and sta= rt > sending reports to the hypervisor that there are new pages that have be= en > freed and are no longer in use. > > An added advantage to this approach is that we should be reducing the > overall memory footprint of the guest as it will be more likely to recy= cle > warm pages versus trying to allocate the reported pages that were likel= y > evicted from the guest memory. > > Since we will only be reporting one zone at a time we keep the boundary= > limited to being defined for just the zone we are currently reporting p= ages > from. Doing this we can keep the number of additional pointers needed q= uite > small. To flag that the boundaries are in place we use a single bit > in the zone to indicate that reporting and the boundaries are active. > > The determination of when to start reporting is based on the tracking o= f > the number of free pages in a given area versus the number of reported > pages in that area. We keep track of the number of reported pages per > free_area in a separate zone specific area. We do this to avoid modifyi= ng > the free_area structure as this can lead to false sharing for the highe= st > order with the zone lock which leads to a noticeable performance > degradation. [...] > + > +/* request page reporting on this zone */ > +void __page_reporting_request(struct zone *zone) > +{ > + struct page_reporting_dev_info *phdev; > + > + rcu_read_lock(); > + > + /* > + * We use RCU to protect the ph_dev_info pointer. In almost all > + * cases this should be present, however in the unlikely case of > + * a shutdown this will be NULL and we should exit. > + */ > + phdev =3D rcu_dereference(ph_dev_info); > + if (unlikely(!phdev)) > + return; > + Just a minor comment here. Although this is unlikely to trigger still I think you should release the= rcu_read_lock before returning. > + /* > + * We can use separate test and set operations here as there > + * is nothing else that can set or clear this bit while we are > + * holding the zone lock. The advantage to doing it this way is > + * that we don't have to dirty the cacheline unless we are > + * changing the value. > + */ > + __set_bit(ZONE_PAGE_REPORTING_REQUESTED, &zone->flags); > + > + /* > + * Delay the start of work to allow a sizable queue to > + * build. For now we are limiting this to running no more > + * than 10 times per second. > + */ > + if (!atomic_fetch_inc(&phdev->refcnt)) > + schedule_delayed_work(&phdev->work, HZ / 10); > + > + rcu_read_unlock(); > +} > + [...] > + } > + > + /* enable page reporting notification */ > + static_branch_enable(&page_reporting_notify_enabled); > +err_out: > + mutex_unlock(&page_reporting_mutex); > + > + return err; > +} > +EXPORT_SYMBOL_GPL(page_reporting_startup); > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org > --=20 Thanks Nitesh