Re: [RFC PATCH v2 0/5] virtio-balloon: Working Set Reporting

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 25, 2023 at 10:20:11PM +0000, T.J. Alumbaugh wrote:
> This is the device implementation for the proposed expanded balloon feature
> described here:
> 
> https://lore.kernel.org/linux-mm/20230509185419.1088297-1-yuanchu@google.com/
> 
> This series has a fixed number of "bins" for the working set report, but this is
> not a constraint of the system. The bin number is fixed at device realization
> time (in other implementations it is specified as a command line argument). Once
> that number is fixed, this determines the correct number of bin intervals to
> pass to the QMP/HMP function 'working_set_config'. Any feedback on how to
> properly construct that function for this use case (passing a variable length
> list?) would be appreciated.

It's been a while. Is there interest is reviving this? I also note that
reserving a feature bit is very much recommended to avoid a complete
mess.


> New in V2:
> =========
> 
> - Patch series is now: header file changes, device changes, QMP changes, HMP
> chagnes, and migration changes.
> 
> - Exmaple usages of QMP and HMP interface are in their respective commit
> messages.
> 
> - "ws" -> "working_set" throughout
> 
> Motivation
> ==========
> As mentioned in the above message, the use case is a host with overcommitted
> memory and 1 or more VMs. The goal is to get both timely and accurate
> information on overall memory utilization in order to drive appropriate
> reclaim activities, since in some client device use cases a VM might need a
> significant fraction of the overall memory for a period of time, but then
> enter a quiet period that results in a large number of cold pages in the
> guest.
> 
> The balloon device now has a number of features to assist in sharing memory
> resources amongst the guests and host (e.g free page hinting, stats, free page
> reporting). As mentioned in slide 12 in [1], the balloon doesn't have a good
> mechanism to drive the reclaim of guest cache. Our use case includes both
> typical page cache as well as "application caches" with memory that should be
> discarded in times of system-wide memory pressure. In some cases, virtio-pmem
> can be a method for host control of guest cache but there are undesirable
> security implications.
> 
> Working Set Reporting
> =====================
> The patch series here includes:
> 
>  - Actual device implementation for VIRTIO_F_WS_REPORTING to standardize the
>    configuration and communication of Working Set reports from the guest. This
>    includes a notification virtqueue for receiving config information and
>    requests for a report (a feature which could be expanded for additional use
>    cases) and a virtqueue for the actual report from the driver.
> 
>  - QMP changes so that a controller program can use the existing QEMU socket
>    mechanism to configure and request WS reports and then read the reports as
>    a JSON property on the balloon.
> 
> Working Set reporting in the balloon provides:
> 
>  - an accurate picture of current memory utilization in the guest
>  - event driven reporting (with configurable rate limiting) to deliver reports
>    during times of memory pressure.
> 
> The reporting mechanism can be combined with a domain-specific balloon policy
> to drive the separate reclaim activities in a coordinated fashion.
> 
> TODOs:
> ======
>  -  A synchronization mechanism must be added to the functions that send WS
>     Config and WS Request, otherwise concurrent callers (through QMP) can mix
>     messages on the virtqueue sending the data to the driver.
> 
>  - The device currently has a hard-coded setting of 4 'bins' for a Working Set
>    report, whereas the specification calls for anywhere between 2 and 16.
> 
>  - A WS_EVENT notification through QMP should include the actual report,
>    whereas right now we query for that information right after a WS_EVENT is
>    received.
> 
> References:
> 
> [1] https://kvmforum2020.sched.com/event/eE4U/virtio-balloonpmemmem-managing-guest-memory-david-hildenbrand-michael-s-tsirkin-red-hat
> 
> T.J. Alumbaugh (5):
>   virtio-balloon: Add Working Set Reporting feature
>   virtio-balloon: device has Working Set Reporting
>   virtio-balloon: Add QMP functions for Working Set
>   virtio-balloon: Add HMP functions for Working Set
>   virtio-balloon: Migration of working set config
> 
>  hmp-commands.hx                               |  26 ++
>  hw/core/machine-hmp-cmds.c                    |  21 ++
>  hw/virtio/virtio-balloon-pci.c                |   2 +
>  hw/virtio/virtio-balloon.c                    | 239 +++++++++++++++++-
>  include/hw/virtio/virtio-balloon.h            |  13 +-
>  include/monitor/hmp.h                         |   2 +
>  .../standard-headers/linux/virtio_balloon.h   |  20 ++
>  include/sysemu/balloon.h                      |   9 +-
>  monitor/monitor.c                             |   1 +
>  qapi/machine.json                             |  66 +++++
>  qapi/misc.json                                |  26 ++
>  softmmu/balloon.c                             |  31 ++-
>  12 files changed, 449 insertions(+), 7 deletions(-)
> 
> -- 
> 2.41.0.rc0.172.g3f132b7071-goog