KVM Archive on lore.kernel.org
 help / color / Atom feed
From: Alexander Duyck <alexander.duyck@gmail.com>
To: Nitesh Narayan Lal <nitesh@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	virtio-dev@lists.oasis-open.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	lcapitulino@redhat.com, Pankaj Gupta <pagupta@redhat.com>,
	"Wang, Wei W" <wei.w.wang@intel.com>,
	Yang Zhang <yang.zhang.wz@gmail.com>,
	Rik van Riel <riel@surriel.com>,
	David Hildenbrand <david@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	dodgen@google.com, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	dhildenb@redhat.com, Andrea Arcangeli <aarcange@redhat.com>,
	john.starks@microsoft.com, Dave Hansen <dave.hansen@intel.com>,
	Michal Hocko <mhocko@suse.com>,
	cohuck@redhat.com
Subject: Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure
Date: Thu, 15 Aug 2019 16:00:46 -0700
Message-ID: <CAKgT0UduKXTMHD2qWqEa7wQPOFYtaQ5Sx3XS9Ki8i8-_kTdmkg@mail.gmail.com> (raw)
In-Reply-To: <6241ef40-9403-1cb0-4e91-a1b86fcf1388@redhat.com>

On Thu, Aug 15, 2019 at 12:23 PM Nitesh Narayan Lal <nitesh@redhat.com> 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 <nitesh@redhat.com> wrote:
> >>> On 8/12/19 2:47 PM, Alexander Duyck wrote:
> >>>> On Mon, Aug 12, 2019 at 6:13 AM Nitesh Narayan Lal <nitesh@redhat.com> 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 <nitesh@redhat.com>
> >>> [...]
> >>>>> +}
> >>>>> +
> >>>>> +/**
> >>>>> + * __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.

  reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-12 13:12 [RFC][PATCH v12 0/2] mm: Support for page reporting Nitesh Narayan Lal
2019-08-12 13:12 ` [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure Nitesh Narayan Lal
2019-08-12 18:47   ` Alexander Duyck
2019-08-12 20:04     ` Nitesh Narayan Lal
2019-08-12 20:05     ` David Hildenbrand
2019-08-13 10:30       ` Nitesh Narayan Lal
2019-08-13 10:34         ` David Hildenbrand
2019-08-13 10:42           ` Nitesh Narayan Lal
2019-08-13 10:44             ` David Hildenbrand
2019-08-13 23:14           ` Alexander Duyck
2019-08-14  7:07             ` David Hildenbrand
2019-08-14 12:49               ` [virtio-dev] " Nitesh Narayan Lal
2019-08-14 15:49     ` Nitesh Narayan Lal
2019-08-14 16:11       ` Alexander Duyck
2019-08-15 13:15         ` Nitesh Narayan Lal
2019-08-15 19:22           ` Nitesh Narayan Lal
2019-08-15 23:00             ` Alexander Duyck [this message]
2019-08-16 18:35               ` Nitesh Narayan Lal
2019-08-12 13:12 ` [RFC][Patch v12 2/2] virtio-balloon: interface to support free page reporting Nitesh Narayan Lal
2019-08-14 10:29   ` Cornelia Huck
2019-08-14 11:47     ` Nitesh Narayan Lal
2019-08-14 13:42       ` Cornelia Huck
2019-08-14 14:01         ` Nitesh Narayan Lal
2019-08-12 13:13 ` [QEMU Patch 1/2] virtio-balloon: adding bit for page reporting support Nitesh Narayan Lal
2019-08-12 13:13   ` [QEMU Patch 2/2] virtio-balloon: support for handling page reporting Nitesh Narayan Lal
2019-08-12 15:18     ` Alexander Duyck
2019-08-12 15:26       ` Nitesh Narayan Lal

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAKgT0UduKXTMHD2qWqEa7wQPOFYtaQ5Sx3XS9Ki8i8-_kTdmkg@mail.gmail.com \
    --to=alexander.duyck@gmail.com \
    --cc=aarcange@redhat.com \
    --cc=cohuck@redhat.com \
    --cc=dave.hansen@intel.com \
    --cc=david@redhat.com \
    --cc=dhildenb@redhat.com \
    --cc=dodgen@google.com \
    --cc=john.starks@microsoft.com \
    --cc=konrad.wilk@oracle.com \
    --cc=kvm@vger.kernel.org \
    --cc=lcapitulino@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=mst@redhat.com \
    --cc=nitesh@redhat.com \
    --cc=pagupta@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=riel@surriel.com \
    --cc=virtio-dev@lists.oasis-open.org \
    --cc=wei.w.wang@intel.com \
    --cc=yang.zhang.wz@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvm/0 kvm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvm kvm/ https://lore.kernel.org/kvm \
		kvm@vger.kernel.org kvm@archiver.kernel.org
	public-inbox-index kvm


Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.kvm


AGPL code for this site: git clone https://public-inbox.org/ public-inbox