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Subject[PATCH v34 0/4] Virtio-balloon: support free page reporting
This patch series is separated from the previous "Virtio-balloon
Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,
implemented by this series enables the virtio-balloon driver to report
hints of guest free pages to the host. It can be used to accelerate live
migration of VMs. Here is an introduction of this usage:

Live migration needs to transfer the VM's memory from the source machine
to the destination round by round. For the 1st round, all the VM's memory
is transferred. From the 2nd round, only the pieces of memory that were
written by the guest (after the 1st round) are transferred. One method
that is popularly used by the hypervisor to track which part of memory is
written is to write-protect all the guest memory.

This feature enables the optimization by skipping the transfer of guest
free pages during VM live migration. It is not concerned that the memory
pages are used after they are given to the hypervisor as a hint of the
free pages, because they will be tracked by the hypervisor and transferred
in the subsequent round if they are used and written.

* Tests
- Test Environment
Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
Guest: 8G RAM, 4 vCPU
Migration setup: migrate_set_speed 100G, migrate_set_downtime 2 second

- Test Results
- Idle Guest Live Migration Time (results are averaged over 10 runs):
- Optimization v.s. Legacy = 284ms vs 1757ms --> ~84% reduction
- Guest with Linux Compilation Workload (make bzImage -j4):
- Live Migration Time (average)
Optimization v.s. Legacy = 1402ms v.s. 2528ms --> ~44% reduction
- Linux Compilation Time
Optimization v.s. Legacy = 5min6s v.s. 5min12s
--> no obvious difference

- mm:
- add a new API max_free_page_blocks, which estimates the max
number of free page blocks that a free page list may have
- get_from_free_page_list: store addresses to multiple arrays,
instead of just one array. This removes the limitation of being
able to report only 2TB free memory (the largest array memory
that can be allocated on x86 is 4MB, which can store 2^19
addresses of 4MB free page blocks).
- virtio-balloon:
- Allocate multiple arrays to load free page hints;
- Use the same method in v32 to do guest/host interaction, the
differeces are
- the hints are tranferred array by array, instead of
one by one.
- send the free page block size of a hint along with the cmd
id to host, so that host knows each address represents e.g.
a 4MB memory in our case.
- mm/get_from_free_page_list: The new implementation to get free page
hints based on the suggestions from Linus:
This avoids the complex call chain, and looks more prudent.
- virtio-balloon:
- use a fix-sized buffer to get free page hints;
- remove the cmd id related interface. Now host can just send a free
page hint command to the guest (via the host_cmd config register)
to start the reporting. Currentlty the guest reports only the max
order free page hints to host, which has generated similar good
results as before. But the interface used by virtio-balloon to
report can support reporting more orders in the future when there
is a need.
- virtio-balloon:
- rename cmd_id_use to cmd_id_active;
- report_free_page_func: detach used buffers after host sends a vq
interrupt, instead of busy waiting for used buffers.
- virtio-balloon:
- virtio_balloon_send_free_pages: return -EINTR rather than 1 to
indicate an active stop requested by host; and add more
comments to explain about access to cmd_id_received without
- add_one_sg: add TODO to comments about possible improvement.
- mm/walk_free_mem_block: add cond_sched() for each order
- mm/page_poison: only expose page_poison_enabled(), rather than more
changes did in v28, as we are not 100% confident about that for now.
- virtio-balloon: use a separate buffer for the stop cmd, instead of
having the start and stop cmd use the same buffer. This avoids the
corner case that the start cmd is overridden by the stop cmd when
the host has a delay in reading the start cmd.
- mm/page_poison: Move PAGE_POISON to page_poison.c and add a function
to expose page poison val to kernel modules.
- add a new patch to expose page_poisoning_enabled to kernel modules
- virtio-balloon: set poison_val to 0xaaaaaaaa, instead of 0xaa
v25->v26: virtio-balloon changes only
- remove kicking free page vq since the host now polls the vq after
initiating the reporting
- report_free_page_func: detach all the used buffers after sending
the stop cmd id. This avoids leaving the detaching burden (i.e.
overhead) to the next cmd id. Detaching here isn't considered
overhead since the stop cmd id has been sent, and host has already
moved formard.
- mm: change walk_free_mem_block to return 0 (instead of true) on
completing the report, and return a non-zero value from the
callabck, which stops the reporting.
- virtio-balloon:
- use enum instead of define for VIRTIO_BALLOON_VQ_INFLATE etc.
- avoid __virtio_clear_bit when bailing out;
- a new method to avoid reporting the some cmd id to host twice
- destroy_workqueue can cancel free page work when the feature is
- fail probe when the free page vq size is less than 2.
- change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
- kick when vq->num_free < half full, instead of "= half full"
- replace BUG_ON with bailing out
- check vb->balloon_wq in probe(), if null, bail out
- add a new feature bit for page poisoning
- solve the corner case that one cmd id being sent to host twice
- change to kick the device when the vq is half-way full;
- open-code batch_free_page_sg into add_one_sg;
- change cmd_id from "uint32_t" to "__virtio32";
- reserver one entry in the vq for the driver to send cmd_id, instead
of busywaiting for an available entry;
- add "stop_update" check before queue_work for prudence purpose for
now, will have a separate patch to discuss this flag check later;
- init_vqs: change to put some variables on stack to have simpler
- add destroy_workqueue(vb->balloon_wq);
- add_one_sg: some code and comment re-arrangement
- send_cmd_id: handle a cornercase

For previous ChangeLog, please reference

Wei Wang (4):
mm: support to get hints of free page blocks
mm/page_poison: expose page_poisoning_enabled to kernel modules

drivers/virtio/virtio_balloon.c | 357 ++++++++++++++++++++++++++++++++----
include/linux/mm.h | 3 +
include/uapi/linux/virtio_balloon.h | 14 ++
mm/page_alloc.c | 82 +++++++++
mm/page_poison.c | 6 +
5 files changed, 426 insertions(+), 36 deletions(-)


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