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From: Nitesh Narayan Lal <nitesh@redhat.com>
To: Alexander Duyck <alexander.duyck@gmail.com>,
	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
Subject: Re: [virtio-dev] [PATCH v6 4/6] mm: Introduce Reported pages
Date: Thu, 22 Aug 2019 12:18:56 -0400	[thread overview]
Message-ID: <91355107-ed73-fce5-7051-3a746b526163@redhat.com> (raw)
In-Reply-To: <20190821145950.20926.83684.stgit@localhost.localdomain>


On 8/21/19 10:59 AM, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> 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 idea
> is that in order for a page to cross from one side of the boundary to the
> other it will need to go through the reporting process. Ultimately a
> free_list has been fully processed when the boundary has been moved from
> 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 start
> sending reports to the hypervisor that there are new pages that have been
> 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 recycle
> warm pages versus trying to allocate the reported pages that were likely
> 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 pages
> from. Doing this we can keep the number of additional pointers needed quite
> 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 of
> 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 modifying
> the free_area structure as this can lead to false sharing for the highest
> 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 = 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);
>
>
> ---------------------------------------------------------------------
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> For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
>
-- 
Thanks
Nitesh


  reply	other threads:[~2019-08-22 16:19 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21 14:59 [PATCH v6 0/6] mm / virtio: Provide support for unused page reporting Alexander Duyck
2019-08-21 14:59 ` [PATCH v6 1/6] mm: Adjust shuffle code to allow for future coalescing Alexander Duyck
2019-08-21 14:59 ` [PATCH v6 2/6] mm: Move set/get_pcppage_migratetype to mmzone.h Alexander Duyck
2019-08-21 14:59 ` [PATCH v6 3/6] mm: Use zone and order instead of free area in free_list manipulators Alexander Duyck
2019-08-21 14:59 ` [PATCH v6 4/6] mm: Introduce Reported pages Alexander Duyck
2019-08-22 16:18   ` Nitesh Narayan Lal [this message]
2019-08-22 16:24     ` [virtio-dev] " Alexander Duyck
2019-08-21 14:59 ` [PATCH v6 5/6] virtio-balloon: Pull page poisoning config out of free page hinting Alexander Duyck
2019-08-21 15:00 ` [PATCH v6 6/6] virtio-balloon: Add support for providing unused page reports to host Alexander Duyck
2019-08-21 15:00 ` [PATCH v6 QEMU 1/3] virtio-ballon: Implement support for page poison tracking feature Alexander Duyck
2019-08-21 15:00 ` [PATCH v6 QEMU 2/3] virtio-balloon: Add bit to notify guest of unused page reporting Alexander Duyck
2019-08-21 15:00 ` [PATCH v6 QEMU 3/3] virtio-balloon: Provide a interface for " Alexander Duyck
2019-08-22 10:43 ` [PATCH v6 0/6] mm / virtio: Provide support " Pankaj Gupta
2019-08-22 15:32   ` Alexander Duyck
2019-08-23  5:16     ` Pankaj Gupta
2019-08-23 15:02       ` Alexander Duyck

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