Re: [PATCH v14 0/6] mm / virtio: Provide support for unused page reporting

[David Hildenbrand]

On 19.11.19 22:46, Alexander Duyck wrote:
> This series provides an asynchronous means of reporting unused guest
> pages to a hypervisor so that the memory associated with those pages can
> be dropped and reused by other processes and/or guests on the host. Using
> this it is possible to avoid unnecessary I/O to disk and greatly improve
> performance in the case of memory overcommit on the host.
> 
> When enabled it will allocate a set of statistics to track the number of
> reported pages. When the nr_free for a given free area is greater than
> this by the high water mark we will schedule a worker to begin pulling the
> non-reported memory and to provide it to the reporting interface via a
> scatterlist.
> 
> Currently this is only in use by virtio-balloon however there is the hope
> that at some point in the future other hypervisors might be able to make
> use of it. In the virtio-balloon/QEMU implementation the hypervisor is
> currently using MADV_DONTNEED to indicate to the host kernel that the page
> is currently unused. It will be zeroed and faulted back into the guest the
> next time the page is accessed.

Remind me why we are using MADV_DONTNEED? Mostly for debugging purposes
right now, right? Did you do any measurements with MADV_FREE? I guess
there should be quite a performance increase in some scenarios.

> 
> To track if a page is reported or not the Uptodate flag was repurposed and
> used as a Reported flag for Buddy pages. We walk though the free list
> isolating pages and adding them to the scatterlist until we either
> encounter the end of the list or have filled the scatterlist with pages to
> be reported. If we fill the scatterlist before we reach the end of the
> list we rotate the list so that the first unreported page we encounter is
> moved to the head of the list as that is where we will resume after we
> have freed the reported pages back into the tail of the list.

So the boundary pointer didn't actually provide that big of a benefit I
assume (IOW, worst thing is you have to re-scan the whole list)?

> 
> Below are the results from various benchmarks. I primarily focused on two
> tests. The first is the will-it-scale/page_fault2 test, and the other is
> a modified version of will-it-scale/page_fault1 that was enabled to use
> THP. I did this as it allows for better visibility into different parts
> of the memory subsystem. The guest is running with 32G for RAM on one
> node of a E5-2630 v3. The host has had some power saving features disabled
> by setting the /dev/cpu_dma_latency value to 10ms.
> 
> Test                page_fault1 (THP)     page_fault2
> Name         tasks  Process Iter  STDEV  Process Iter  STDEV
> Baseline         1    1203934.75  0.04%     379940.75  0.11%
>                 16    8828217.00  0.85%    3178653.00  1.28%
> 
> Patches applied  1    1207961.25  0.10%     380852.25  0.25%
>                 16    8862373.00  0.98%    3246397.25  0.68%
> 
> Patches enabled  1    1207758.75  0.17%     373079.25  0.60%
>  MADV disabled  16    8870373.75  0.29%	   3204989.75  1.08%
> 
> Patches enabled  1    1261183.75  0.39%     373201.50  0.50%
>                 16    8371359.75  0.65%    3233665.50  0.84%
> 
> Patches enabled  1    1090201.50  0.25%     376967.25  0.29%
>  page shuffle   16    8108719.75  0.58%    3218450.25  1.07%
> 
> The results above are for a baseline with a linux-next-20191115 kernel,
> that kernel with this patch set applied but page reporting disabled in
> virtio-balloon, patches applied but the madvise disabled by direct
> assigning a device, the patches applied and page reporting fully
> enabled, and the patches enabled with page shuffling enabled.  These
> results include the deviation seen between the average value reported here
> versus the high and/or low value. I observed that during the test memory
> usage for the first three tests never dropped whereas with the patches
> fully enabled the VM would drop to using only a few GB of the host's
> memory when switching from memhog to page fault tests.
> 
> Most of the overhead seen with this patch set enabled seems due to page
> faults caused by accessing the reported pages and the host zeroing the page
> before giving it back to the guest. This overhead is much more visible when
> using THP than with standard 4K pages. In addition page shuffling seemed to
> increase the amount of faults generated due to an increase in memory churn.

MADV_FREE would be interesting.

> 
> The overall guest size is kept fairly small to only a few GB while the test
> is running. If the host memory were oversubscribed this patch set should
> result in a performance improvement as swapping memory in the host can be
> avoided.
> 
> A brief history on the background of unused page reporting can be found at:
> https://lore.kernel.org/lkml/29f43d5796feed0dec8e8bb98b187d9dac03b900.camel@xxxxxxxxxxxxxxx/
> 
> Changes from v12:
> https://lore.kernel.org/lkml/20191022221223.17338.5860.stgit@xxxxxxxxxxxxxxxxxxxxx/
> Rebased on linux-next 20191031
> Renamed page_is_reported to page_reported
> Renamed add_page_to_reported_list to mark_page_reported
> Dropped unused definition of add_page_to_reported_list for non-reporting case
> Split free_area_reporting out from get_unreported_tail
> Minor updates to cover page
> 
> Changes from v13:
> https://lore.kernel.org/lkml/20191105215940.15144.65968.stgit@xxxxxxxxxxxxxxxxxxxxx/
> Rewrote core reporting functionality
>   Merged patches 3 & 4
>   Dropped boundary list and related code
>   Folded get_reported_page into page_reporting_fill
>   Folded page_reporting_fill into page_reporting_cycle
> Pulled reporting functionality out of free_reported_page
>   Renamed it to __free_isolated_page
>   Moved page reporting specific bits to page_reporting_drain
> Renamed phdev to prdev since we aren't "hinting" we are "reporting"
> Added documentation to describe the usage of unused page reporting
> Updated cover page and patch descriptions to avoid mention of boundary
> 
> 
> ---
> 
> Alexander Duyck (6):
>       mm: Adjust shuffle code to allow for future coalescing
>       mm: Use zone and order instead of free area in free_list manipulators
>       mm: Introduce Reported pages
>       mm: Add unused page reporting documentation
>       virtio-balloon: Pull page poisoning config out of free page hinting
>       virtio-balloon: Add support for providing unused page reports to host
> 
> 
>  Documentation/vm/unused_page_reporting.rst |   44 ++++
>  drivers/virtio/Kconfig                     |    1 
>  drivers/virtio/virtio_balloon.c            |   88 +++++++
>  include/linux/mmzone.h                     |   56 +----
>  include/linux/page-flags.h                 |   11 +
>  include/linux/page_reporting.h             |   31 +++
>  include/uapi/linux/virtio_balloon.h        |    1 
>  mm/Kconfig                                 |   11 +
>  mm/Makefile                                |    1 
>  mm/memory_hotplug.c                        |    2 
>  mm/page_alloc.c                            |  181 +++++++++++----
>  mm/page_reporting.c                        |  337 ++++++++++++++++++++++++++++
>  mm/page_reporting.h                        |  125 ++++++++++
>  mm/shuffle.c                               |   12 -
>  mm/shuffle.h                               |    6 
>  15 files changed, 805 insertions(+), 102 deletions(-)

So roughly 100 LOC less added, that's nice to see :)

I'm planning to look into the details soon, just fairly busy lately. I
hope Mel Et al. can also comment.

-- 
Thanks,

David / dhildenb