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

[David Hildenbrand <david@redhat.com>]

On 26.11.19 17:45, Alexander Duyck wrote:
> On Tue, 2019-11-26 at 13:20 +0100, David Hildenbrand wrote:
>> 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.
> 
> There are actually a few reasons for not using MADV_FREE.
> 
> The first one was debugging as I could visibly see how much memory had
> been freed by just checking the memory consumption by the guest. I didn't
> have to wait for memory pressure to trigger the memory freeing. In
> addition it would force the pages out of the guest so it was much easier
> to see if I was freeing the wrong pages.
> 
> The second reason is because it is much more portable. The MADV_FREE has
> only been a part of the Linux kernel since about 4.5. So if you are
> running on an older kernel the option might not be available.

I guess optionally enabling it (for !filebacked and !huge pages) in QEMU
after sensing would be possible. Fallback to ram_discard_range().

> 
> The third reason is simply effort involved. If I used MADV_DONTNEED then I
> can just use ram_block_discard_range which is the same function used by
> other parts of the balloon driver.

Yes, that makes perfect sense.

> 
> Finally it is my understanding is that MADV_FREE only works on anonymous
> memory (https://elixir.bootlin.com/linux/v5.4/source/mm/madvise.c#L700). I
> was concerned that using MADV_FREE wouldn't work if used on file backed
> memory such as hugetlbfs which is an option for QEMU if I am not mistaken.

Yes, MADV_FREE works just like MADV_DONTNEED only on anonymous memory.
In case of files/hugetlbfs you have to use

fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, ...).

E.g., see qemu/exec.c:ram_block_discard_range. You can do something
similar to this:


static bool madv_free_sensed, madv_free_available;
int ret = -EINVAL;

/*
 * MADV_FREE only works on anonymous memory, and especially not on
 * hugetlbfs. Older kernels don't support it.
 */
if (rb->page_size == qemu_host_page_size && rb->fb != -1 &&
    (!madv_free_sensed || madv_free_available)) {
    ret = madvise(start, length, MADV_FREE);
    if (ret) {
	madv_free_sensed = true;
        madv_free_available = false;
    } else if (!madv_free_sensed) {
        madv_free_sensed = true;
	madv_free_available = true;
    }
}

/* fallback to MADV_DONTNEED / FALLOC_FL_PUNCH_HOLE */
if (ret) {
    ram_block_discard_range(rb, start, length);
}


I agree that something like should be an addon to the current patch set.

> 
>>> 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)?
> 
> I rewrote the code quite a bit to get rid of the disadvantages.
> Specifically what the boundary pointer was doing was saving our place in
> the list when we left. Even without that we still had to re-scan the
> entire list with each zone processed anyway. With these changes we end up
> potentially having to perform one additional rescan per free list.
> 
> Where things differ now is that the fetching function doesn't bail out of
> the list and start over per page. Instead it fills the entire scatterlist
> before it exits, and before doing so it will advance the head to the next
> non-reported page in the list. In addition instead of walking all of the
> orders and migrate types looking for each page the code is now more
> methodical and will only work one free list at a time and do not revisit
> it until we have processed the entire zone.

Makes perfect sense to me.

> 
> Even with all that we still take a pretty significant performance hit in
> the page shuffing case, however I am willing to give that up for the sake
> of being less intrusive.

Makes sense as well, especially for a first version.

> 
>>> 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.
> 
> I can probably code something up. However that is going to push a bunch of
> complexity into the QEMU code and doesn't really mean much to the kernel
> code. I can probably add it as another QEMU patch to the set since it is
> just a matter of having a function similar to ram_block_discard_range that
> uses MADV_FREE instead of MADV_DONTNEED.

Yes, addon patch makes perfect sense. The nice thing about MADV_FREE is
that you only take back pages from a process when really under memory
pressure (before going to SWAP). You will still get a pagefault on the
next access (to identify that the page is still in use after all), but
don't have to fault in a fresh page.

> 
>>> 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@linux.intel.com/
>>>
>>> Changes from v12:
>>> https://lore.kernel.org/lkml/20191022221223.17338.5860.stgit@localhost.localdomain/
>>> 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@localhost.localdomain/
>>> 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.
> 
> Agreed. I can see if I can generate something to get the MADV_FREE
> numbers. I suspect they were probably be somewhere between the MADV
> disabled and fully enabled case, since we will still be taking the page
> faults but not doing the zeroing.

Exactly.

-- 
Thanks,

David / dhildenb