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Date:   Wed, 24 Jul 2019 15:24:35 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Nitesh Narayan Lal <nitesh@redhat.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>, kvm@vger.kernel.org,
        david@redhat.com, dave.hansen@intel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, 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: [PATCH v2 0/5] mm / virtio: Provide support for page hinting
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On Wed, Jul 24, 2019 at 02:40:02PM -0400, Nitesh Narayan Lal wrote:
> 
> On 7/24/19 12:54 PM, Alexander Duyck wrote:
> > This series provides an asynchronous means of hinting to a hypervisor
> > that a guest page is no longer in use and can have the data associated
> > with it dropped. To do this I have implemented functionality that allows
> > for what I am referring to as page hinting
> >
> > The functionality for this is fairly simple. When enabled it will allocate
> > statistics to track the number of hinted pages in a given free area. When
> > the number of free pages exceeds this value plus a high water value,
> > currently 32,
> Shouldn't we configure this to a lower number such as 16?
> >  it will begin performing page hinting which consists of
> > pulling pages off of free list and placing them into a scatter list. The
> > scatterlist is then given to the page hinting device and it will perform
> > the required action to make the pages "hinted", in the case of
> > virtio-balloon this results in the pages being madvised as MADV_DONTNEED
> > and as such they are forced out of the guest. After this they are placed
> > back on the free list, and an additional bit is added if they are not
> > merged indicating that they are a hinted buddy page instead of a standard
> > buddy page. The cycle then repeats with additional non-hinted pages being
> > pulled until the free areas all consist of hinted pages.
> >
> > I am leaving a number of things hard-coded such as limiting the lowest
> > order processed to PAGEBLOCK_ORDER,
> Have you considered making this option configurable at the compile time?
> >  and have left it up to the guest to
> > determine what the limit is on how many pages it wants to allocate to
> > process the hints.
> It might make sense to set the number of pages to be hinted at a time from the
> hypervisor.
> >
> > My primary testing has just been to verify the memory is being freed after
> > allocation by running memhog 79g on a 80g guest and watching the total
> > free memory via /proc/meminfo on the host. With this I have verified most
> > of the memory is freed after each iteration. As far as performance I have
> > been mainly focusing on the will-it-scale/page_fault1 test running with
> > 16 vcpus. With that I have seen at most a 2% difference between the base
> > kernel without these patches and the patches with virtio-balloon disabled.
> > With the patches and virtio-balloon enabled with hinting the results
> > largely depend on the host kernel. On a 3.10 RHEL kernel I saw up to a 2%
> > drop in performance as I approached 16 threads,
> I think this is acceptable.
> >  however on the the lastest
> > linux-next kernel I saw roughly a 4% to 5% improvement in performance for
> > all tests with 8 or more threads. 
> Do you mean that with your patches the will-it-scale/page_fault1 numbers were
> better by 4-5% over an unmodified kernel?
> > I believe the difference seen is due to
> > the overhead for faulting pages back into the guest and zeroing of memory.
> It may also make sense to test these patches with netperf to observe how much
> performance drop it is introducing.
> > Patch 4 is a bit on the large side at about 600 lines of change, however
> > I really didn't see a good way to break it up since each piece feeds into
> > the next. So I couldn't add the statistics by themselves as it didn't
> > really make sense to add them without something that will either read or
> > increment/decrement them, or add the Hinted state without something that
> > would set/unset it. As such I just ended up adding the entire thing as
> > one patch. It makes it a bit bigger but avoids the issues in the previous
> > set where I was referencing things before they had been added.
> >
> > Changes from the RFC:
> > https://lore.kernel.org/lkml/20190530215223.13974.22445.stgit@localhost.localdomain/
> > Moved aeration requested flag out of aerator and into zone->flags.
> > Moved bounary out of free_area and into local variables for aeration.
> > Moved aeration cycle out of interrupt and into workqueue.
> > Left nr_free as total pages instead of splitting it between raw and aerated.
> > Combined size and physical address values in virtio ring into one 64b value.
> >
> > Changes from v1:
> > https://lore.kernel.org/lkml/20190619222922.1231.27432.stgit@localhost.localdomain/
> > Dropped "waste page treatment" in favor of "page hinting"
> We may still have to try and find a better name for virtio-balloon side changes.
> As "FREE_PAGE_HINT" and "PAGE_HINTING" are still confusing.

Right. In fact I'm not sure why should these be called hints at all.
VIRTIO_BALLOON_F_FREE_PAGE_HINT is a hint because pages might no
longer be free by the time they are reported.

I think of this one as a free page reporting capability.
I don't really mind how are internal kernel functions called,
but I think for virtio uapi things that's a better name.


> > Renamed files and functions from "aeration" to "page_hinting"
> > Moved from page->lru list to scatterlist
> > Replaced wait on refcnt in shutdown with RCU and cancel_delayed_work_sync
> > Virtio now uses scatterlist directly instead of intermedate array
> > Moved stats out of free_area, now in seperate area and pointed to from zone
> > Merged patch 5 into patch 4 to improve reviewability
> > Updated various code comments throughout
> >
> > ---
> >
> > Alexander Duyck (5):
> >       mm: Adjust shuffle code to allow for future coalescing
> >       mm: Move set/get_pcppage_migratetype to mmzone.h
> >       mm: Use zone and order instead of free area in free_list manipulators
> >       mm: Introduce Hinted pages
> >       virtio-balloon: Add support for providing page hints to host
> >
> >
> >  drivers/virtio/Kconfig              |    1 
> >  drivers/virtio/virtio_balloon.c     |   47 ++++++
> >  include/linux/mmzone.h              |  116 ++++++++------
> >  include/linux/page-flags.h          |    8 +
> >  include/linux/page_hinting.h        |  139 ++++++++++++++++
> >  include/uapi/linux/virtio_balloon.h |    1 
> >  mm/Kconfig                          |    5 +
> >  mm/Makefile                         |    1 
> >  mm/internal.h                       |   18 ++
> >  mm/memory_hotplug.c                 |    1 
> >  mm/page_alloc.c                     |  238 ++++++++++++++++++++--------
> >  mm/page_hinting.c                   |  298 +++++++++++++++++++++++++++++++++++
> >  mm/shuffle.c                        |   24 ---
> >  mm/shuffle.h                        |   32 ++++
> >  14 files changed, 796 insertions(+), 133 deletions(-)
> >  create mode 100644 include/linux/page_hinting.h
> >  create mode 100644 mm/page_hinting.c
> >
> > --
> -- 
> Thanks
> Nitesh